Fix various warnings

[tatokis]

This fixes almost all warnings in the codebase when compiling on Linux under gcc 9.3.

Only one I couldn't figure out, which is possibly a false compiler warning, is

qprojectm_mainwindow.cpp: In member function ‘void QProjectM_MainWindow::updateFilteredPlaylist(const QString&)’:
qprojectm_mainwindow.cpp:1258:68: warning: ‘activePresetId’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 1258 |    if (activePresetId.hasValue() && data.id == activePresetId.value()) {
      |                                                ~~~~~~~~~~~~~~~~~~~~^~

It complains about activePresetId being uninitialized, however it's a class instance and the default constructor is called.
activePresetId.value() above won't be called if it's never been assigned (as hasValue() only returns true if operator= has been called).

[revmischa]

Wow cool!