Merge: gamnit & sdl2: implement app::audio using SDL2 mixer

Intro a decent implementation of `app::audio` for desktop computers using SDL2 mixer. It supports WAV, MP3, OGG and more. The underlying SDL2 mixer bindings are minimal, they offer only the required features, more can be added as needed.

Clients of the gamnit framework will need to `apt-get install libsdl2-mixer-dev`.

Pull-Request: #2466