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)
BTW -- tests are currently failing on Ubuntu and Windows:
http://builds.mixxx.org/jenkins/job/master-prs/141/testReport/

On Mon, Oct 20, 2014 at 1:51 AM, Marcos Cardinot <notifications@github.com>
wrote:

> In src/mixxx.cpp:
>
> > @@ -817,6 +839,8 @@ void MixxxMainWindow::onNewSkinLoaded() {
> >                               ConfigKey("[Microphone]", "show_microphone"));
> >      setVisibilityOptionState(m_pViewShowPreviewDeck,
> >                               ConfigKey("[PreviewDeck]", "show_previewdeck"));
> > +    setVisibilityOptionState(m_pViewShowCoverArt,
>
> done!
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/mixxxdj/mixxx/pull/278/files#r19068787>.
>