Cover art support

http://mixxx.org/wiki/doku.php/cover_art_support

- [x] implement database scheme
- [x] search for embedded cover
- [x] search for cover in track directory
- [x] save covers in disk cache
- [x] search for covers in disk cache
- [x] minimal ui (display covers in lower left corner)
- [x] handle existing users
- [x] LRU cache
- [x] unit tests
- [x] md5 hash
- [x] do not show cover widget for not supported features:
    - banshee
    - browse
    - itunes
    - recordings
    - rhythmbox
    - traktor

**Supported features:**
- analysis
- autodj
- crates
- library
- history
- playlist

**Skin for tests:**
- Deere1280x800-WXGA
this is not a `const` that we use in the whole class so please write it in lower case