Customize system menu items through dedicated API (#677)

Document new interface changes.

src/modules/powerrename/dll/dllmain.cpp

@@ -227,6 +227,9 @@ public:
         return 0;
     }
 
+    virtual void register_system_menu_helper(PowertoySystemMenuIface* helper) override { }
+    virtual void signal_system_menu_action(const wchar_t* name) override { }
+
     // Destroy the powertoy and free memory
     virtual void destroy() override
     {