Merge Desktop SoundEffect into main SoundEffect, AudioLoader to Desktop/

This merges the SoundEffect found in `Desktop/Audio/SoundEffect.cs` into `Audio/SoundEffect.cs`. It also moves `AudioLoader.cs` to `Desktop/Audio/`, since the Windows and Linux versions were essentially the same file.

This should basically be the exact same code for all platforms, except now it's in single files and not several duplicates.

The only projects affected are Linux and WindowsGL.