Turn the private #define for the group-shifting modifier into API

Add GDK_MODIFIER_INTENT_SHIFT_GROUP to enum GdkModifierIntent
and handle it in gdk_keymap_get_modifier_mask(). Add an X11
impl of the method and return keymap_x11->group_switch_mask.
Return 0 from the default impl because we don't know.

gdk/quartz/gdkkeys-quartz.c

@@ -757,6 +757,9 @@ gdk_quartz_keymap_get_modifier_mask (GdkKeymap         *keymap,
     case GDK_MODIFIER_INTENT_NO_TEXT_INPUT:
       return GDK_MOD2_MASK | GDK_CONTROL_MASK;
 
+    case GDK_MODIFIER_INTENT_SHIFT_GROUP:
+      return GDK_MOD1_MASK;
+
     default:
       g_return_val_if_reached (0);
     }