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/display.c

@@ -129,6 +129,15 @@ typedef struct
 
 G_DEFINE_TYPE(MetaDisplay, meta_display, G_TYPE_OBJECT);
 
+/* Signals */
+enum
+{
+  OVERLAY_KEY,
+  LAST_SIGNAL
+};
+
+static guint display_signals [LAST_SIGNAL] = { 0 };
+
 /**
  * The display we're managing.  This is a singleton object.  (Historically,
  * this was a list of displays, but there was never any way to add more
@@ -168,6 +177,14 @@ MetaGroup*     get_focussed_group (MetaDisplay *display);
 static void
 meta_display_class_init (MetaDisplayClass *klass)
 {
+  display_signals[OVERLAY_KEY] =
+    g_signal_new ("overlay-key",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_LAST,
+                  0,
+                  NULL, NULL,
+                  g_cclosure_marshal_VOID__VOID,
+                  G_TYPE_NONE, 0);  
 }
 
 /**
@@ -5167,6 +5184,12 @@ meta_display_remove_autoraise_callback (MetaDisplay *display)
     }
 }
 
+void
+meta_display_overlay_key_activate (MetaDisplay *display)
+{
+  g_signal_emit (display, display_signals[OVERLAY_KEY], 0);
+}
+
 #ifdef HAVE_COMPOSITE_EXTENSIONS
 void
 meta_display_get_compositor_version (MetaDisplay *display,