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!)

src/sandbox.rs

@@ -1,6 +1,5 @@
 use crate::{
-    Application, Clipboard, Color, Command, Element, Error, Settings,
-    Subscription,
+    Application, Color, Command, Element, Error, Settings, Subscription,
 };
 
 /// A sandboxed [`Application`].
@@ -162,11 +161,7 @@ where
         T::title(self)
     }
 
-    fn update(
-        &mut self,
-        message: T::Message,
-        _clipboard: &mut Clipboard,
-    ) -> Command<T::Message> {
+    fn update(&mut self, message: T::Message) -> Command<T::Message> {
         T::update(self, message);
 
         Command::none()