mpd: Create unique names for all items in browse

Replace the playlist uri/name mapping with a mapping for all the items
in browse. Check the mapping when browsing a path.

This has the added benefit of serving as a cache for browse, so we don't
need to traverse the path parts and lookup each of them for each call to
browse.
Merge pull request #762 from trygveaa/fix/mpd-browse-duplicate

mpd: Create unique names for all items in browse