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
yes always prefer metadata. Lets just make this a design rule. This is also the intended way to save a cover per track in every other application that I know. Plus id3v2 was made exactly for that purpose.

> Yes, this is defiantly an issue, imagine a 30 Tracks 2 CD Sampler. This
could be solved by storing and comparing a MD5 checksum.

Since we search the image asynchronous it is easy to calculate a MD5 checksum as well. Marcos can you make a wiki-page/gDocs explaining what possible situations we can have with normal album folders and samplers/collections. We already have a few different scenarios this in PR. Then we can write what a user should expect for each situation and check what we have to do to full fill these expectations.