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/include/prefs.h

@@ -247,6 +247,8 @@ void meta_prefs_get_window_binding (const char          *name,
                                     unsigned int        *keysym,
                                     MetaVirtualModifier *modifiers);
 
+void meta_prefs_get_overlay_binding (MetaKeyCombo *combo);
+
 typedef enum
 {
   META_VISUAL_BELL_INVALID = 0,