main menu: Add the ability to hide and reorder the main menu items.

To change the shown menu items add the line "root_menu_order:<items>" into your config.cfg
<items> can be any of:
bookmarks, files, database, wps, settings, recording, radio, playlists, plugins, system_menu, shortcuts

Manual entry by Alexander Levin

Change-Id: Ie7f4bfb0f795184de094d05fc341a6cedd1c0cde
Reviewed-on: http://gerrit.rockbox.org/104
Reviewed-by: Jonathan Gordon <rockbox@jdgordon.info>

apps/settings.h

@@ -844,6 +844,8 @@ struct user_settings
 #endif
 
     char start_directory[MAX_PATHNAME+1];
+    /* status setting for the root menu customisability. 0 = default, 1 = loaded from cfg */
+    int root_menu;
 };
 
 /** global variables **/