window-props: Change a warning to a debug message.

XAppStatusIcon creates a dummy GdkWindow at the status icon's
position in order to satisfy gtk_menu_popup_at_rect(), but muffin
complains because this dummy window shows up as the popup menu's
transient parent and isn't recognized.

Since this is all necessary to make sure XApp popups show up in
the correct place, stop spamming the logs over it.

This also affects the systray applet for both tooltips and menus.

Author: mtwebster

src/x11/window-props.c

@@ -1748,8 +1748,9 @@ reload_transient_for (MetaWindow    *window,
                                                  transient_for);
       if (!parent)
         {
-          meta_warning ("Invalid WM_TRANSIENT_FOR window 0x%lx specified for %s.\n",
-                        transient_for, window->desc);
+          meta_topic (META_DEBUG_WINDOW_STATE,
+                      "Invalid WM_TRANSIENT_FOR window 0x%lx specified for %s.\n",
+                      transient_for, window->desc);
           transient_for = None;
         }
       else if (parent->override_redirect)