Fix song editor visualization after maximizing

[PhysSong]

Fixes #70 by overriding changeEvent in the SongEditorWindow class.

[zonkmachine]

I get a crash introduced in 86f9a75.

To reproduce.

1. Open a project
2. Hit reload

Basically the crash happens if there is anything in the Song Editor. Just click in the project to get a TCO and click "New Project" is enough.

I don't know why I didn't see this when I tested the PR.

Thread 1 (Thread 0x7ffff7f2c900 (LWP 22555)):
#0  0x00007ffff71258a9 in QWidgetPrivate::setWSGeometry(bool, QRect const&) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
---Type <return> to continue, or q <return> to quit---
#1  0x00007ffff7125a7e in QWidgetPrivate::setWSGeometry(bool, QRect const&) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#2  0x00007ffff7125a7e in QWidgetPrivate::setWSGeometry(bool, QRect const&) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#3  0x00007ffff712e0df in QWidgetPrivate::setGeometry_sys(int, int, int, int, bool) ()
   from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#4  0x00007ffff70d9784 in QWidget::resize(QSize const&) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#5  0x00007ffff70d9acc in QWidget::setMaximumSize(int, int) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#6  0x00007ffff70d9d9f in QWidget::setFixedHeight(int) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#7  0x00000000005fa585 in TrackContainerView::realignTracks (this=0x1225550)
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/gui/TrackContainerView.cpp:232
        content = 0x121a450
#8  0x0000000000625cbd in SongEditorWindow::changeEvent (this=0x1214410, event=0x7fffffffb510)
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/gui/editors/SongEditor.cpp:731
No locals.
#9  0x00007ffff70e17bb in QWidget::event(QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#10 0x00007ffff74c59b3 in QMainWindow::event(QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#11 0x00007ffff708afdc in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
   from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#12 0x00007ffff7091f16 in QApplication::notify(QObject*, QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#13 0x00007ffff691790d in QCoreApplication::notifyInternal(QObject*, QEvent*) ()
   from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
No symbol table info available.
#14 0x00007ffff70db507 in QWidget::overrideWindowState(QFlags<Qt::WindowState>) ()
   from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
---Type <return> to continue, or q <return> to quit---
No symbol table info available.
#15 0x00007ffff74dab6d in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#16 0x00007ffff74e2b71 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#17 0x00007ffff74e41e7 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#18 0x00007ffff692c010 in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) ()
   from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
No symbol table info available.
#19 0x00007ffff70898ab in QApplication::focusChanged(QWidget*, QWidget*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#20 0x00007ffff708fe2c in QApplicationPrivate::setFocusWidget(QWidget*, Qt::FocusReason) ()
   from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#21 0x00007ffff70dcc8b in QWidget::setFocus(Qt::FocusReason) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#22 0x0000000000593197 in QWidget::setFocus (this=0x1225550) at /usr/include/qt4/QtGui/qwidget.h:432
No locals.
#23 0x00000000005884fa in TrackContentObjectView::~TrackContentObjectView (this=0x1994f00, __in_chrg=<optimized out>)
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/core/Track.cpp:307
No locals.
#24 0x0000000000669bbe in PatternView::~PatternView (this=0x1994f00, __in_chrg=<optimized out>)
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/tracks/Pattern.cpp:618
No locals.
#25 0x0000000000669c00 in PatternView::~PatternView (this=0x1994f00, __in_chrg=<optimized out>)
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/tracks/Pattern.cpp:620
No locals.
#26 0x00007ffff69302b1 in QObjectPrivate::deleteChildren() () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
No symbol table info available.
#27 0x00007ffff70e12a2 in QWidget::~QWidget() () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
No symbol table info available.
#28 0x000000000058c154 in TrackContentWidget::~TrackContentWidget (this=0x1914da0, __in_chrg=<optimized out>)
---Type <return> to continue, or q <return> to quit---
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/core/Track.cpp:1094
No locals.
#29 0x0000000000591a9a in TrackView::~TrackView (this=0x1914cc0, __in_chrg=<optimized out>)
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/core/Track.cpp:2569
No locals.
#30 0x0000000000661a36 in InstrumentTrackView::~InstrumentTrackView (this=0x1914cc0, __in_chrg=<optimized out>)
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/tracks/InstrumentTrack.cpp:977
No locals.
#31 0x0000000000661a80 in InstrumentTrackView::~InstrumentTrackView (this=0x1914cc0, __in_chrg=<optimized out>)
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/tracks/InstrumentTrack.cpp:983
No locals.
#32 0x00000000005fa9d5 in TrackContainerView::clearAllTracks (this=0x1225550)
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/gui/TrackContainerView.cpp:331
        tv = 0x1914cc0
        t = 0x7ffff7e653e0
#33 0x0000000000580f7f in Song::clearProject (this=0xc9a200)
    at /home/zonkmachine/builds/lmms-stable-1.2/lmms/src/core/Song.cpp:864
No locals.

Edit @PhysSong: Removed irrelevant part from the backtrace

[PhysSong]

@zonkmachine Neither Qt4 nor Qt5 gives me the crash.

[zonkmachine]

Recompiled with Qt5 and it doesn't see this issue. I don't know if a recent update of my linuxmint installation could have done this. I don't think Qt4 gets any updates anywhere but maybe some native package Qt depends on was changed? I'll keep looking. If it's just me crashing we're good... ;)

[zonkmachine]

Yeah. I'm on Xenial (linuxMint 18.3) and the libx11 packages received security updates.
Installed versions went 2:1.6.3-1ubuntu2 -> 2:1.6.3-1ubuntu2.1