browser: make sorting optional

This adds a new option, "noop", 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.

src/enums.h

@@ -27,7 +27,7 @@ enum class SpaceAddMode { AddRemove, AlwaysAdd };
 std::ostream &operator<<(std::ostream &os, SpaceAddMode sam);
 std::istream &operator>>(std::istream &is, SpaceAddMode &sam);
 
-enum class SortMode { Name, ModificationTime, CustomFormat };
+enum class SortMode { Name, ModificationTime, CustomFormat, NoOp };
 std::ostream &operator<<(std::ostream &os, SortMode sm);
 std::istream &operator>>(std::istream &is, SortMode &sm);