Make Command implementations platform-specific

This allows us to introduce a platform-specific `Action` to both `iced_native`
and `iced_web` and remove the `Clipboard` from `Application::update` to maintain
purity.

Additionally, this should let us implement further actions to let users query
and modify the shell environment (e.g. window, clipboard, and more!)

glutin/src/application.rs

@@ -48,7 +48,7 @@ where
 
     let subscription = application.subscription();
 
-    runtime.spawn(init_command);
+    application::run_command(init_command, &mut runtime);
     runtime.track(subscription);
 
     let context = {
@@ -211,7 +211,6 @@ async fn run_instance<A, E, C>(
                         &mut application,
                         &mut runtime,
                         &mut debug,
-                        &mut clipboard,
                         &mut messages,
                     );