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/window.rs

@@ -1,4 +1,6 @@
 //! Build window-based GUI applications.
+mod action;
 mod event;
 
+pub use action::Action;
 pub use event::Event;