add Last-Modified property to tracks

To make sorting by modification date possible #671
add setting for Last-Modified album sorting #671
add Last-Modified to AlbumCache and CachedMPD

see #671
add Last-Modified to Album classes

see #671
add Last-Modified to AlbumsFragment (actually do sort)

see #671
Tell MusicBuilder about MPD's "directory" keyword

The info MPD has given for a "directory" was added to the "file"
in the list before.
This fix is needed for #671 because directories have Last-Modified
Bit of a hack because Tools.getRanges is hardly curable.