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/application.rs

@@ -1,7 +1,5 @@
 use crate::window;
-use crate::{
-    Clipboard, Color, Command, Element, Executor, Menu, Settings, Subscription,
-};
+use crate::{Color, Command, Element, Executor, Menu, Settings, Subscription};
 
 /// An interactive cross-platform application.
 ///
@@ -80,7 +78,7 @@ use crate::{
 ///         String::from("A cool application")
 ///     }
 ///
-///     fn update(&mut self, _message: Self::Message, _clipboard: &mut Clipboard) -> Command<Self::Message> {
+///     fn update(&mut self, _message: Self::Message) -> Command<Self::Message> {
 ///         Command::none()
 ///     }
 ///
@@ -129,11 +127,7 @@ pub trait Application: Sized {
     /// this method.
     ///
     /// Any [`Command`] returned will be executed immediately in the background.
-    fn update(
-        &mut self,
-        message: Self::Message,
-        clipboard: &mut Clipboard,
-    ) -> Command<Self::Message>;
+    fn update(&mut self, message: Self::Message) -> Command<Self::Message>;
 
     /// Returns the event [`Subscription`] for the current state of the
     /// application.
@@ -249,14 +243,9 @@ where
 {
     type Renderer = crate::renderer::Renderer;
     type Message = A::Message;
-    type Clipboard = iced_winit::Clipboard;
 
-    fn update(
-        &mut self,
-        message: Self::Message,
-        clipboard: &mut iced_winit::Clipboard,
-    ) -> Command<Self::Message> {
-        self.0.update(message, clipboard)
+    fn update(&mut self, message: Self::Message) -> Command<Self::Message> {
+        self.0.update(message)
     }
 
     fn view(&mut self) -> Element<'_, Self::Message> {
@@ -329,12 +318,8 @@ where
         self.0.title()
     }
 
-    fn update(
-        &mut self,
-        message: Self::Message,
-        clipboard: &mut Clipboard,
-    ) -> Command<Self::Message> {
-        self.0.update(message, clipboard)
+    fn update(&mut self, message: Self::Message) -> Command<Self::Message> {
+        self.0.update(message)
     }
 
     fn subscription(&self) -> Subscription<Self::Message> {

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()