Fix runtime dependencies of LibraryScanner

[uklotzde]

Extracted from #2511 (without URI changes) that should fix the test failures in #2508.

• Disable LibraryScanner during library tests
• Remove dependencies on TrackCollection from LibraryScanner
• Use Qt::DirectConnection for signal-to-signal connections

TODO (see code): The LibraryScanner should be extracted and decoupled from TrackCollectionManager in the long term.

[uklotzde]

AppVeyor CI failure is unrelated: EngineSyncTest.RateChangeTest (Failed)

[daschuer]

This has also an empty library here.
It turns out that the issue already exists in Master.
I am investigating.

[uklotzde]

We didn't receive any reports about database issues in master. So my first guess would be that this might be caused by local circumstances. Nevertheless we should try to find the cause. Unfortunately no one else without experiencing those issues can help here.

[daschuer]

Got it. It was caused by #2470
My database schema was already at 31. I have manually set the version number down to 30 and now my tracks are back.

[daschuer]

This one works and looks good. Thank you.

[uklotzde]

@daschuer That's the manual intervention I mentioned in #2514 that will still needed. We cannot detect this reliably. Developers need to be aware of this when testing PRs that contain a database migration with a preliminary version number. The final version number may differ if other PRs with schema changes get merged earlier.