Implement client window flashing

This will be used in python to grab the player's attention on PMs or other
interesting events.

Sources/Plasma/NucleusLib/pnMessage/plClientMsg.h

@@ -94,6 +94,8 @@ public:
         kEnableRenderScene,
         kResetGraphicsDevice,
         kSetGraphicsDefaults,
+
+        kFlashWindow,
     };
 
     // graphics settings fields