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!)

native/src/command/action.rs

@@ -0,0 +1,24 @@
+use crate::clipboard;
+use crate::window;
+
+pub enum Action<T> {
+    Future(iced_futures::BoxFuture<T>),
+    Clipboard(clipboard::Action<T>),
+    Window(window::Action),
+}
+
+impl<T> Action<T> {
+    /// Applies a transformation to the result of a [`Command`].
+    pub fn map<A>(self, f: impl Fn(T) -> A + 'static + Send + Sync) -> Action<A>
+    where
+        T: 'static,
+    {
+        use iced_futures::futures::FutureExt;
+
+        match self {
+            Self::Future(future) => Action::Future(Box::pin(future.map(f))),
+            Self::Clipboard(action) => Action::Clipboard(action.map(f)),
+            Self::Window(window) => Action::Window(window),
+        }
+    }
+}