playlist: Simplify playlist modified detection

Any modifications to the playlist (insert, delete, shuffle, etc)
will cause the modified flag to be set. The flag is cleared when
the playlist is saved. Code that generates playlists can manually
clear the modified flag if appropriate; there is now a proper API
for this so the tagcache and pictureflow don't need to resort to
hacks.

Change-Id: I8d3c723265a41db07a13de3f1d2abb0444528d57

apps/plugin.h

@@ -954,6 +954,7 @@ struct plugin_api {
     bool (*tagtree_subentries_do_action)(bool (*action_cb)(const char *file_name));
 #endif
     void (*adjust_volume)(int steps);
+    void (*playlist_set_modified)(struct playlist_info *playlist, bool modified);
 };
 
 /* plugin header */