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/action.rs

@@ -0,0 +1,18 @@
+/// An operation to be performed on some window.
+#[derive(Debug)]
+pub enum Action {
+    /// Resize the window.
+    Resize {
+        /// The new logical width of the window
+        width: u32,
+        /// The new logical height of the window
+        height: u32,
+    },
+    /// Move the window.
+    Move {
+        /// The new logical x location of the window
+        x: i32,
+        /// The new logical y location of the window
+        y: i32,
+    },
+}

native/src/window/event.rs

@@ -13,10 +13,9 @@ pub enum Event {
 
     /// A window was resized.
     Resized {
-        /// The new width of the window (in units)
+        /// The new logical width of the window
         width: u32,
-
-        /// The new height of the window (in units)
+        /// The new logical height of the window
         height: u32,
     },