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
Ok you are doing this right after you started mixxx when the cache is cold I assume.

Once the cache is hot I don't see any speed problems anymore. I also don't notice anything when the cache is cold but the picture is already saved in the DB or when it is saved in the id3tag. I'm not 100% sure with the tags because only have the images in the id3tags for a very limited number of tracks.

I do get a stalling UI but from what I can see this happens when I select tracks where the cover is not saved in the id3tag and we need to scan the folder.  Can you make test and report what you experience with hot vs cold cache and if it makes a difference if you run on a fresh config or one where you already have selected most of the tracks once. Can you also post what machine you are using for testing, i guess interesting is SSD vs HDD and the number of logical CPU-cores.

In my case @cardinot should try to investigate what could cause a UI-freeze when we have to scan a directory.  If this really only happens for new tracks I would like to know what the real bottleneck is maybe it is easier to fix then using timer slots. More of a cure the cause not the symptoms approach.