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/pane_grid/src/main.rs

@@ -1,8 +1,7 @@
 use iced::{
     button, executor, keyboard, pane_grid, scrollable, Align, Application,
-    Button, Clipboard, Color, Column, Command, Container, Element,
-    HorizontalAlignment, Length, PaneGrid, Row, Scrollable, Settings,
-    Subscription, Text,
+    Button, Color, Column, Command, Container, Element, HorizontalAlignment,
+    Length, PaneGrid, Row, Scrollable, Settings, Subscription, Text,
 };
 use iced_native::{event, subscription, Event};
 
@@ -51,11 +50,7 @@ impl Application for Example {
         String::from("Pane grid - Iced")
     }
 
-    fn update(
-        &mut self,
-        message: Message,
-        _clipboard: &mut Clipboard,
-    ) -> Command<Message> {
+    fn update(&mut self, message: Message) -> Command<Message> {
         match message {
             Message::Split(axis, pane) => {
                 let result = self.panes.split(