Mostly ID tag and performance issues

See commit msgs for details.

- Next/prev should be faster because of async next/prev preview and album art loading.
- filters stays persistent
- Use ID tag for album art
- Use mp3agic for ID3 tag read (native does not really its job on various files)
- Dont read track length in advance, this is time consuming, in particular for VBRs (use the player itself)
- Album Art lookup at runtime, inkl. cache for fs lookup
- More info at current playing screen

I added mp3agic as a submodule, see devinfo/ProjectSetup.txt for how to get this to work.