Settings menu: add main menu customization

Lets you launch the main_menu_config plugin
from Settings, so that it remains accessible,
even after all other menu items have been
hidden.

Includes minor changes to the main menu config
plugin, so that it better fits in with the rest
of the settings menus:

* Keep theme enabled
* Autosave
* Add "On" or "Off" suffix to each item, to prevent
  confusion when Icon_Config is blank, as is the case
  with certain iconsets
* Update current selection after swapping items
* Prefix spoken selection with row number for enhanced
  clarity when swapping items
* Change title to LANG_MAIN_MENU
* ACTION_STD_OK to toggle an item's visibility
* ACTION_STD_CONTEXT to access options for reordering
  menu items, or for reverting to the default menu
* Ask for confirmation before reverting

Change-Id: I74240b94243224c76e23ef8f3a0559bd8ba28df0

manual/plugins/main_menu_config.tex

@@ -1,17 +1,2 @@
 \subsection{Main Menu Configuration}
-{\label{ref:main_menu_config}}
-
-This plugin helps you customizing the main menu (i.e. reorder or hide menu
-items). It changes the appropriate configuration file as described in
-\reference{ref:CustomisingTheMainMenu}.
-
-When you start the plugin, the available main menu items will be displayed.
-By pressing \ActionStdOk{} you open a menu with the following options:
-\begin{description}
-\item[Toggle Item] Hide the selected menu item or make it visible again
-\item[Move Item Up] Swap the selected menu item with the previous one
-\item[Move Item down] Swap the selected menu item with the next one
-\item[Load Default Configuration] Discards all customization
-\item[Exit] Save your changes to the configuration file and exit the plugin
-\end{description}
-You can leave the plugin without saving by pressing \ActionStdCancel{}.
+see \reference{ref:main_menu_config}