Add meta_window_is_on_primary_monitor function

This is useful in a couple of places to avoid opencoding it.

https://bugzilla.gnome.org/show_bug.cgi?id=609258

src/core/window.c

@@ -3213,6 +3213,17 @@ meta_window_is_fullscreen (MetaWindow *window)
   return window->fullscreen;
 }
 
+/**
+ * meta_window_is_on_primary_monitor:
+ *
+ * Return value: %TRUE if the window is on the primary monitor
+ */
+gboolean
+meta_window_is_on_primary_monitor (MetaWindow *window)
+{
+  return window->monitor->number == window->screen->primary_monitor_index;
+}
+
 void
 meta_window_tile (MetaWindow *window)
 {

src/meta/window.h

@@ -126,6 +126,7 @@ void     meta_window_foreach_ancestor         (MetaWindow            *window,
 
 MetaMaximizeFlags meta_window_get_maximized (MetaWindow *window);
 gboolean          meta_window_is_fullscreen (MetaWindow *window);
+gboolean          meta_window_is_on_primary_monitor (MetaWindow *window);
 
 gboolean meta_window_is_mapped (MetaWindow  *window);
 gboolean meta_window_toplevel_is_mapped (MetaWindow  *window);