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!)

winit/src/window.rs

@@ -0,0 +1,7 @@
+pub use iced_native::window::*;
+
+/// The window of an [`Application`].
+///
+/// [`Application`]: crate::Application
+#[derive(Debug)]
+pub struct Window {}