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
>I'm almost sure that running all SoundSource and SoundSourceProxy stuff would be much slower than just loading it directly doing QImage(location)

Test it, I can't see why it should be much slower. Besides it doesn't matter because we load the images asynchronous. 

> We could also handle cases when the image is too small and cases when the image is duplicated... anyway, what I mean is that having a disk-cache is essential to embedded covers.

actually all of that could be solved by using the database as well. I don't see any reason why we couldn't do it without. Using the in-disk cache makes the code easier in some places but that is pretty much it.

>We were doing it before, however, IMHO "cover" (and "folder") might not be a common cover_name for all Mixxx users.

I'm german and I use cover and folder. If there is a semi standart set for this the we can follow we should to it. Doesn't cost us much and a lot of people will be happy that we automatically do what they expect. If we don't find a cover.*/folder.* we can fall back to your current solution.