Add option to set MPD top-level path. Only tracks that are under this path will be used to populate artist and album listings. This is mainly of use when MPD is configured to use a UPnP backend.
BUG: 591


git-svn-id: http://cantata.googlecode.com/svn/trunk@5639 b1e49159-3266-923f-9c3e-04f16369770a