Change shell_global_get_modifier_keys to shell_global_get_pointer.

We can't use gdk_display_get_pointer/gdk_window_get_pointer from gjs
when XKB is active. We already had a wrapper that did the
get-modifier-state part of that, but some places also need the
get-pointer-location part of it. So update our wrapper to return both,
and update js code to use it.

https://bugzilla.gnome.org/show_bug.cgi?id=613428

js/ui/altTab.js

@@ -173,7 +173,7 @@ AltTabPopup.prototype = {
         // https://bugzilla.gnome.org/show_bug.cgi?id=596695 for
         // details.) So we check now. (Have to do this after updating
         // selection.)
-        let mods = global.get_modifier_keys();
+        let [x, y, mods] = global.get_pointer();
         if (!(mods & Gdk.ModifierType.MOD1_MASK)) {
             this._finish();
             return false;