Cover art support

**More information about the project:**
http://mixxx.org/wiki/doku.php/cover_art_support

**Skin for tests:**
- Deere1280x800-WXGA

**Supported features:**
- analysis
- autodj
- crates
- library
- history
- playlist

**First Part**
- [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

**Second Part**
https://github.com/cardinot/mixxx/pull/1
- [x] redesign dlgtrackinfo to show covers
- [x] load cover in dlgtrackinfo
- [x] edit cover location in dlgtrackinfo
- [x] unset cover (load default)
- [x] reload cover from metadata
- [x] warning box issue
- [x] icon for each cover_art action
- [x] copy cover from external dir to track dir (if the user changes the cover)

**Third Part**
https://github.com/cardinot/mixxx/pull/2
- [x] add cover_art column in Library feature
- [x] add cover_art column in AutoDJ feature
- [x] add cover_art column in Analyze feature
- [x] add cover_art column in Playlists feature
- [x] add cover_art column in Crates feature
- [x] adjust cover art size
- [x] delay cover loading when the user is scrolling fast
- [x] batch updates (avoiding doing many writes in a short time)
QtConcurrent isn't very suitable for use in Mixxx -- it suffers from a variety of problems (single application-wide thread pool for one). Instead, this class should be a polling worker queue -- since that's much simpler and easier to reason about.