Add natural sorting for tracks and filters

[guillermofbriceno]

Firstly I'd like to say I've been looking for a music player for Linux like this one for years, so thanks for your work!

At the moment, Fooyin uses ASCII-order sorting for the library, as is also the case for most other players I've used. For a music player, I believe natural sorting should be used instead. The core argument for this goes pretty far back, but in our case, consider the current sorting of some Bach works with their commonly used Opus number:

BWV 1
BWV 10
BWV 100
...

Using QCollator instead, the player will use a natural sort more similar to the behavior of foobar2000:

BWV 1
BWV 2
BWV 3
...

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I am a C programmer by trade so there might be some bad code here.

[ludouzi]

Thanks for the contribution! I agree that natural sorting should be preferred for user-facing strings and cases such as this.

The changes look good here. I wasn't aware of QCollator before now, so thanks for bringing it to my attention. LibraryTree also uses a simple locale-aware sort and should be switched over to use QCollator, but I'll do so separately.