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
You can use a Qt Timer but it has the "disadvantage" to be exact an does not respect other threads. 

An other solution you will find here 
https://github.com/mixxxdj/mixxx/blob/master/src/controlindicator.cpp

If you like to be in sync with the waveform renderer, you may take a look here: 
https://github.com/mixxxdj/mixxx/blob/master/src/widget/wspinny.cpp#L201
https://github.com/mixxxdj/mixxx/blob/4c9cda48faa28512408a90cfb51cb487787f7c49/src/waveform/waveformwidgetfactory.cpp#L225