wayland: Reduce warning level for parentless temporaries

Temp windows without parent are used frequently in our testsuite;
using g_warning on them causes the tests to fail, which is not
useful. Reduce the warning to a g_message.

gdk/wayland/gdkwindow-wayland.c

@@ -1288,7 +1288,7 @@ should_map_as_popup (GdkWindow *window)
             return TRUE;
         }
       else
-        g_warning ("Window %p is a temporary window without parent, "
+        g_message ("Window %p is a temporary window without parent, "
                    "application will not be able to position it on screen.",
                    window);
     }