Cover art support

- implement database scheme
- search for embedded cover
- search for cover in track directory
- save covers in disk cache
- search for covers in disk cache
- minimal ui (display covers in lower left corner)
- handle existing users
- LRU cache
>Even for downloaded covers, I personally would prefer to store them along with the tracks. This way they are available for other apps as well. But this should be a preference option. .. and we must handle the case that the disk is full.

No we don't need to handle a full-disk. Our analysis results take a hell lot more space then the covers and when your home folder is so full that you can't even save a <100kb cover anymore you'll have other problems then running mixxx. You also use banshee, they store the covers in `~/cache/media-art` for my complete library that takes about 66MB.

>The disk cache for all covers has also the drawback that it behavior might not be expected. 
If a folder contains a "cover.jpg" it is copied to the cache if it found the first time. 
If the user exchanges the cover, Mixxx is still displaying the cover in the cache.

That is not different from how we treat id3tags currently. Mixxx doesn't notice if you change them automatically the user has to manually reread them. We could to the same for covers.

> 2. ./.jpp

what is that supposed to mean?  I actually like the priority range that Marcos currently has.

> It could be also interesting to specify a cover cache from an external player as cover source.

Maybe but it shouldn't be part of this PR. It is already large enough just covering the internals.