Add an "Above_Tab" pseudo-keysym

We want switching between the windows of an application to be an easily
accessible operation. The convenient and memorable keybinding is the
key above the tab key - but the keysym for that key isn't consistent
across different keyboard layouts.

Add code that figures out the key from the XKB geometry and a magic
keysym name "Above_Tab" that refers to this key and switch
the default binding for cycle_group to <Alt>Above_Tab. (This will
have no effect for the normal case of getting the key binding from
GConf until this patch is applied to Metacity as well.)

https://bugzilla.gnome.org/show_bug.cgi?id=635569

src/core/display-private.h

@@ -221,6 +221,7 @@ struct _MetaDisplay
   KeySym *keymap;
   int keysyms_per_keycode;
   XModifierKeymap *modmap;
+  unsigned int above_tab_keycode;
   unsigned int ignored_modifier_mask;
   unsigned int num_lock_mask;
   unsigned int scroll_lock_mask;
@@ -433,4 +434,7 @@ void meta_display_remove_autoraise_callback (MetaDisplay *display);
 
 void meta_display_overlay_key_activate (MetaDisplay *display);
 
+/* In above-tab-keycode.c */
+guint meta_display_get_above_tab_keycode (MetaDisplay *display);
+
 #endif