CMake changes for Qt5.
Port code to Qt5 and start replacing Phonon with QtMultimedia.

* Compiles and runs
* Is able to play music
* All other playback related things are broken
Port all playback related code except synth to Qt5Multimedia.
Port synth, but I don't hear anything yet.
Move Synth and BufferPlayer implementation to a .cc file.
Synth now plays sounds with piano keys.

However, note synth doesn't work and there are buffer underflows.
Fix note synth.
Implement song metadata fetching.
Remove unused cmake modules.