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.