Add support for a "meta key" which initiates extended WM operations

This patch adds the concept of a special key for WM operations, and
the default is Super_L, which on extended PC hardware is the
"Windows key".  What we do is handle the special case of a press
and release of this key (without any other intervening keys).

Super_L+<key> should still be passed to applications.  In the future
we may want to also take some of these keybindings (e.g. Super+TAB)
though.

http://bugzilla.gnome.org/show_bug.cgi?id=563047

src/core/screen.c

@@ -2980,6 +2980,13 @@ meta_screen_get_screen_number (MetaScreen *screen)
   return screen->number;
 }
 
+/**
+ * meta_screen_get_display:
+ * Retrieve the display associated with screen.
+ * @screen: A #MetaScreen
+ * 
+ * Returns: (transfer none): Display 
+ */
 MetaDisplay *
 meta_screen_get_display (MetaScreen *screen)
 {