feat: somewhere to place extra actions by platform

I have view iced-sckt forked iced, and add the extra actions. and there
do are some extra actions, like set margin for layer-shell, set lock for
ext-session-shell. I think add an any will be of help maybe

winit/src/application.rs

@@ -861,6 +861,7 @@ pub fn run_command<A, C, E>(
                     .send_event(tagger(Ok(())))
                     .expect("Send message to event loop");
             }
+            command::Action::PlatformSpecific(_) => unimplemented!(),
         }
     }
 }