prefernces/sound accept video/* mimetypes

[nephele]

rationale: for one destinquishing video/mp4 and audio/mp4 is really non-trivial and has little gain, which is why we don't do it (see hrev53541) we currently just assign video/mp4 to whaever looks like a mp4 container.

Another thing is that it should be completely valid for someone to use an actuall video for their music, a recording of a song sung by a choir comes to mind, these usually include a video track and stripping those out by users is non-trivial aswell.

In addtion to this the application should set the correct filter when opening tracker instead of poping up "this is not an audio file" after one has been picked. (#15439)