keybindings: Grab and emit a signal when XK_ISO_Next_Group is pressed

This will make it possible to implement input source switching in
gnome-shell using the popular modifiers-only keybinding that's
implemented on the X server through an XKB option.

https://bugzilla.gnome.org/show_bug.cgi?id=697002

src/core/display-private.h

@@ -237,6 +237,8 @@ struct _MetaDisplay
   unsigned int meta_mask;
   MetaKeyCombo overlay_key_combo;
   gboolean overlay_key_only_pressed;
+  MetaKeyCombo *iso_next_group_combos;
+  int n_iso_next_group_combos;
   
   /* Monitor cache */
   unsigned int monitor_cache_invalidated : 1;
@@ -456,6 +458,7 @@ void meta_display_overlay_key_activate (MetaDisplay *display);
 void meta_display_accelerator_activate (MetaDisplay *display,
                                         guint        action,
                                         guint        deviceid);
+gboolean meta_display_modifiers_accelerator_activate (MetaDisplay *display);
 
 /* In above-tab-keycode.c */
 guint meta_display_get_above_tab_keycode (MetaDisplay *display);