Add playback of filesystem folders

Add a new UI fragment to play all audio files within a specific folder.
The implementation is derived from the existing genre and playlist
functionality. The music directories are discovered by querying
EXTERNAL_CONTENT_URI for file paths and extracting the parent directory.

The "Add to home screen" menu item is currently not shown when playing
folders.  Adding this is certainly possible, but requires a rework of
ShortcutActivity to cope with ID strings, instead of numeric IDs (since
we can't generate numeric IDs for filesystem folders).

All other functionality (folder list, songs-within-folder list,
playback, add-to-queue, shuffling, ...) should work fine.