Add meta_window_is_mapped, remove usage of window-private.h from mutter-window.c

mutter-window.c originally grew an #include "window-private.h" for
window->override_redirect, but that was just fixed.  However since
then it also ended up relying on a few other minor private bits.

To fix that, add meta_window_is_mapped, promote meta_window_toplevel_is_mapped
to public, and use the public MetaDisplay accessor.

src/core/window.c

@@ -2321,6 +2321,18 @@ unmap_client_window (MetaWindow *window,
     return FALSE;
 }
 
+/**
+ * meta_window_is_mapped:
+ * @window: a #MetaWindow
+ *
+ * Determines whether the X window for the MetaWindow is mapped.
+ */
+gboolean
+meta_window_is_mapped (MetaWindow  *window)
+{
+  return window->mapped;
+}
+
 /**
  * meta_window_toplevel_is_mapped:
  * @window: a #MetaWindow