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

src/shell-global.h

@@ -82,7 +82,10 @@ GSList       *shell_global_get_monitors        (ShellGlobal  *global);
 GdkRectangle *shell_global_get_primary_monitor (ShellGlobal  *global);
 GdkRectangle *shell_global_get_focus_monitor   (ShellGlobal  *global);
 
-GdkModifierType shell_global_get_modifier_keys (ShellGlobal *global);
+void shell_global_get_pointer (ShellGlobal         *global,
+                               int                 *x,
+                               int                 *y,
+                               ClutterModifierType *mods);
 
 ClutterModifierType shell_get_event_state (ClutterEvent *event);