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.rs

@@ -0,0 +1,77 @@
+mod action;
+
+pub use action::Action;
+
+use std::future::Future;
+
+/// A set of asynchronous actions to be performed by some runtime.
+pub enum Command<T> {
+    None,
+    Single(Action<T>),
+    Batch(Vec<Action<T>>),
+}
+
+impl<T> Command<T> {
+    /// Creates an empty [`Command`].
+    ///
+    /// In other words, a [`Command`] that does nothing.
+    pub fn none() -> Self {
+        Self::None
+    }
+
+    /// Creates a [`Command`] that performs the action of the given future.
+    pub fn perform<A>(
+        future: impl Future<Output = T> + 'static + Send,
+        f: impl Fn(T) -> A + 'static + Send,
+    ) -> Command<A> {
+        use iced_futures::futures::FutureExt;
+
+        Command::Single(Action::Future(Box::pin(future.map(f))))
+    }
+
+    /// Applies a transformation to the result of a [`Command`].
+    pub fn map<A>(
+        self,
+        f: impl Fn(T) -> A + 'static + Send + Sync + Clone,
+    ) -> Command<A>
+    where
+        T: 'static,
+    {
+        match self {
+            Self::None => Command::None,
+            Self::Single(action) => Command::Single(action.map(f)),
+            Self::Batch(batch) => Command::Batch(
+                batch
+                    .into_iter()
+                    .map(|action| action.map(f.clone()))
+                    .collect(),
+            ),
+        }
+    }
+
+    /// Creates a [`Command`] that performs the actions of all the given
+    /// commands.
+    ///
+    /// Once this command is run, all the commands will be executed at once.
+    pub fn batch(commands: impl IntoIterator<Item = Command<T>>) -> Self {
+        let mut batch = Vec::new();
+
+        for command in commands {
+            match command {
+                Self::None => {}
+                Self::Single(command) => batch.push(command),
+                Self::Batch(commands) => batch.extend(commands),
+            }
+        }
+
+        Self::Batch(batch)
+    }
+
+    pub fn actions(self) -> Vec<Action<T>> {
+        match self {
+            Self::None => Vec::new(),
+            Self::Single(action) => vec![action],
+            Self::Batch(batch) => batch,
+        }
+    }
+}