playlist: Optimize playlist_remove_all_tracks()

This was insanely slow for large playlists. Implement it as a
constant-time operation to avoid the O(N!) blowup, and add a
new "clear" command to the control file to avoid flooding it
with individual delete commands.

Change-Id: Ied82d1cba015591edd583f2a2ff6772108f4e176

apps/playlist.h

@@ -47,6 +47,7 @@ enum playlist_command {
     PLAYLIST_COMMAND_SHUFFLE,
     PLAYLIST_COMMAND_UNSHUFFLE,
     PLAYLIST_COMMAND_RESET,
+    PLAYLIST_COMMAND_CLEAR,
     PLAYLIST_COMMAND_COMMENT
 };