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)
I think that the requestPixmap calls are not a big deal when these calls are just trying to get what is already loaded in cache... also, we have already algorithms to delay these calls...

Actually just the future setting up would be done in the main thread, however it is very useful to do not create tons of new instances to load and search... it affects directly the performance...

but maybe you have a smarter idea of how to sort it out... ?