Make sorting in browser optional

This adds a new option, "unsorted", to browser_sort_mode. If this mode
is selected, no sorting is done in the browser view, and the elements
are shown in the same order as received from the MPD server.

This is desirable because the MPD server may list the elements in a significant order which is different from the sort modes of ncmpcpp. An example is mopidy with mopidy-spotify, where the playlists are listed in the same order as they are sorted in Spotify. This change makes it possible to view them in that order in ncmpcpp.