Rockbox

This is the first step of being able to use the pictureflow plugin from a database search. The database now indexes the path to the coverimage for each audio file. In the database browser you can select "Cover by..." genre, artist, composer, year or all and will be presented with a list of cover image paths. (Imagine a call to the pictureflow plugin at this stage, so instead of a list of boring paths you get nice and shiny images). After selecting a path it will display the list of files that would be queued when a cover image is picked in the plugin.

Next step is to work out how to safely call the plugin without losing the context and utilise the return value from it to queue the files.

Issues so far are:

- the preferred size of 100x100 is hardcoded (this should be configurable or at least be consistent with the value in the pictureflow plugin?)
- the db update for covers has the same restrictions as the other tags, i.e. it will only detect changed cover art on initialise
- the coverflow plugin needs to work while music is playing (or stop playback if needs be)
- since there is no guaranteed relation between album name and image file, the album name can't be displayed