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!)

examples/url_handler/src/main.rs

@@ -1,6 +1,6 @@
 use iced::{
-    executor, Application, Clipboard, Command, Container, Element, Length,
-    Settings, Subscription, Text,
+    executor, Application, Command, Container, Element, Length, Settings,
+    Subscription, Text,
 };
 use iced_native::{
     event::{MacOS, PlatformSpecific},
@@ -34,11 +34,7 @@ impl Application for App {
         String::from("Url - Iced")
     }
 
-    fn update(
-        &mut self,
-        message: Message,
-        _clipboard: &mut Clipboard,
-    ) -> Command<Message> {
+    fn update(&mut self, message: Message) -> Command<Message> {
         match message {
             Message::EventOccurred(event) => {
                 if let Event::PlatformSpecific(PlatformSpecific::MacOS(