PlaylistEdit: postpone UpdateQueuedSong() when adding multiple songs

Implement a "bulk" edit mode that postpones both UpdateQueuedSong()
and OnModified().  This way, the playlist version gets incremented
only once.  More importantly: when adding multiple songs to a queue
that consists of only one song, the first song that got added will
always be played next.  By postponing this choice, all newly added
songs get a chance to become the next song.  Fixes the second (and
last) part of Mantis ticket 0004005.

Makefile.am

@@ -181,6 +181,7 @@ src_mpd_SOURCES = \
 	src/PlaylistInfo.hxx \
 	src/PlaylistDatabase.cxx src/PlaylistDatabase.hxx \
 	src/PlaylistUpdate.cxx \
+	src/BulkEdit.hxx \
 	src/IdTable.hxx \
 	src/Queue.cxx src/Queue.hxx \
 	src/QueuePrint.cxx src/QueuePrint.hxx \