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
>What is the cover load priority strategy?

>IMHO the embedded cover should have higher priority. 
>It should be preferred over a cover.jpg in the same folder.

Yes, we are doing it now...
The main concept is: all covers are stored in a disk-cache. So, when we are looking for a cover for the first time, firstly we'll try getting a embedded one. If we do not find any embedded cover, we'll look in the track directory. After that, we save the cover in our disk-cache.

In this way, the very first thing that we have to do is check  if the current requested cover is stored in our disk-cache and after that we could start look into embedded covers and to cover stored in a track directory.
1 - disk cache (just to check if we already got a cover in a past cover request for the current track)
2 - embedded cover
3 - track directory