playlist: Rewrite playlist_save(), optimization & fixes

playlist_save() was a poorly thought out mess. This fixes the
glaring issues and hopefully ensures that saving the playlist
never loses state (such as queued tracks or modified status)
after save+resume.

Indices are now updated on the fly, which is faster and needs
no extra memory. But if an error occurs, the playlist will be
corrupted. There is currently no attempt to handle this since
errors should be unlikely, but some error handling needs to be
added in the future.

Change-Id: If8a5dbd6a596460be08ee0b7bab9f24337886ea4

apps/menus/playlist_menu.c

@@ -83,7 +83,7 @@ int save_playlist_screen(struct playlist_info* playlist)
                                        playlist ? playlist->filename :
                                        playlist_get_current()->filename))
     {
-        playlist_save(playlist, temp, NULL, 0);
+        playlist_save(playlist, temp);
 
         /* reload in case playlist was saved to cwd */
         reload_directory();