Desktop: centre melonDS's 800x600 window

AutoPlace on its own left the resized window in the top-left corner; with policy="center" it
opens in the middle of the desktop. Tried in a running desktop (labwc --reconfigure).

tools/linuxfs/desktop/rc.xml

@@ -21,11 +21,11 @@
   <mouse><doubleClickTime>500</doubleClickTime></mouse>
   <!-- Programs opened on the desktop at a size that suits it. melonDS remembers a 1x window for the
        front end's full-screen toggle (bannerlator-pad-defaults), too small for a desktop: here it
-       opens at 800x600, centred. Only the desktop reads this file, so the front end is unaffected. -->
+       opens at 800x600, centred (AutoPlace alone kept the top-left corner). Only the desktop reads this file, so the front end is unaffected. -->
   <windowRules>
     <windowRule title="melonDS*">
       <action name="ResizeTo" width="800" height="600"/>
-      <action name="AutoPlace"/>
+      <action name="AutoPlace" policy="center"/>
     </windowRule>
   </windowRules>
 </labwc_config>