Drop QtMultimedia and QtMultimediaWidgets deps

[ferdnyc]

Turns out, we don't actually use any of the classes in QtMultimedia (or its Widgets), so this PR drops them as dependencies.

This also prevents both Qt5Network and Qt5OpenGL getting pulled in as dependencies, and reduces our Qt dependency fingerprint significantly to just the three modules (now) listed in the CMakeLists.txt: QtCore, QtGui, and QtWidgets.

Shared libs being what they are, it only reduces the final size of libopenshot.so by about 33Kb, but the size isn't the point. (Besides, not loading four big Qt modules could give runtime a teeny tiny bit more pep, theoretically.)

# Before
$ ldd libopenshot.so.0.2.5 |& grep -i qt
	libQt5MultimediaWidgets.so.5 => /lib64/libQt5MultimediaWidgets.so.5 (0x00007f8911972000)
	libQt5Widgets.so.5 => /lib64/libQt5Widgets.so.5 (0x00007f89112cc000)
	libQt5Multimedia.so.5 => /lib64/libQt5Multimedia.so.5 (0x00007f89111e2000)
	libQt5Gui.so.5 => /lib64/libQt5Gui.so.5 (0x00007f8910c79000)
	libQt5Network.so.5 => /lib64/libQt5Network.so.5 (0x00007f8910af0000)
	libQt5Core.so.5 => /lib64/libQt5Core.so.5 (0x00007f89105d3000)
	libQt5OpenGL.so.5 => /lib64/libQt5OpenGL.so.5 (0x00007f890dfa7000)

# After
$ ldd libopenshot.so.0.2.5 |& grep -i qt
	libQt5Widgets.so.5 => /lib64/libQt5Widgets.so.5 (0x00007f8429fff000)
	libQt5Gui.so.5 => /lib64/libQt5Gui.so.5 (0x00007f842785c000)
	libQt5Core.so.5 => /lib64/libQt5Core.so.5 (0x00007f8427341000)

[codecov]

Codecov Report

Merging #551 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #551   +/-   ##
========================================
  Coverage    48.77%   48.77%           
========================================
  Files          129      129           
  Lines        10036    10036           
========================================
  Hits          4895     4895           
  Misses        5141     5141           

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[ferdnyc]

Committing, as these classes are undeniably not used in the code, so they serve no purpose in the build.

The only way this could cause problems is if we have some kind of implicit dependency on Qt5OpenGL or Qt5Network — and if so (it's highly unlikely), then good! Let's break things like that, so we find them now instead of it causing problems later. We can put anything here that's actually required back, but as a declared explicit dependency.