Implement focus_next operation

... as well as a `count_focusable` composable helper!

src/lib.rs

@@ -192,22 +192,26 @@ use iced_wgpu as renderer;
 use iced_glow as renderer;
 
 pub use iced_native::theme;
+pub use runtime::event;
+pub use runtime::subscription;
 
 pub use application::Application;
 pub use element::Element;
 pub use error::Error;
+pub use event::Event;
 pub use executor::Executor;
 pub use renderer::Renderer;
 pub use result::Result;
 pub use sandbox::Sandbox;
 pub use settings::Settings;
+pub use subscription::Subscription;
 pub use theme::Theme;
 
 pub use runtime::alignment;
 pub use runtime::futures;
 pub use runtime::{
     Alignment, Background, Color, Command, ContentFit, Font, Length, Padding,
-    Point, Rectangle, Size, Subscription, Vector,
+    Point, Rectangle, Size, Vector,
 };
 
 #[cfg(feature = "system")]

src/widget.rs

@@ -211,3 +211,14 @@ pub use qr_code::QRCode;
 #[cfg(feature = "svg")]
 #[cfg_attr(docsrs, doc(cfg(feature = "svg")))]
 pub use svg::Svg;
+
+use crate::Command;
+use iced_native::widget::operation;
+
+/// Focuses the next focusable widget.
+pub fn focus_next<Message>() -> Command<Message>
+where
+    Message: 'static,
+{
+    Command::widget(operation::focus_next())
+}