Sort files in mixxx.qrc.
Add schema.xml to mixxx.qrc.
Load schema.xml from resource path instead of filesystem.

Eliminates the possibility of mismatch between schema.xml and compiled
binary.
Remove now-unnecessary config-path setting from library tests.
Fix SchemaManager backwards-incompatibility check.
SchemaManager improvements.

* Replace integer error codes with an enum.
* Remove now-impossible errors due to loading schema.xml out of the
  binary resources.
* Add a warning case for using a backwards-incompatible library.
Add unit tests for SchemaManager.

Check that we can:
* Successfully parse the current schema.xml from Qt resources.
* Upgrade a fresh database to the binary's required schema version.
* That backwards compatible libraries are OK.
* That backwards incompatible libraries are detected.
* That upgrade failures are successfully recognized.
Remove schema.xml from packaging.
Shade: Added Box around deck cover
Add quick effect super knobs to controlpickermenu
VCI400: config updates
Delete obsolete include directives
New SoundSource/AudioSource API

This is the first step towards a better/cleaner SoundSource API.
The operations for reading audio data have been moved into the
new abstract base class AudioSource.

Currently SoundSource inherits from AudioSource for technical reasons.
This is too much much coupling that needs to be broken up. I have
already some ideas on how to resolve it, but first this work needs to
be integrated.

During the refactoring I've noticed, that the quality and maturity of the
audio decoders for different file types greatly varies:

Good/mature:
- FLAC
- Ogg
- Opus
- WavPack
- libsndfile

Mediocre:
- MP3

Ugly/inefficient:
- M4A
- ModPlug

Untested AudioSource API:
- Apple CoreAudio
- Windows Media Foundation

Unfinished AudioSource API (just a hack):
- FFmpeg

Especially the code for the M4A decoder should be revised and cleaned up as
soon as possible, followed by a review of the MP3 decoder. Reading audio
data must work as stable and efficient as possible. This has priority over
the improvement of the internal software architecture!
Rewrite of the M4A decoder without legacy code
Slightly simplify the analysis of tracks
Initial cleanup of SoundSourceMp3 code
More SoundSourceMp3 optimizations and improvements
Move track metadata properties from SoundSource into separate DTO class
Fix missing FFmpeg initialization in unit tests
Move audio properties from SoundSource to AudioSource