Introduce custom method to widget::Operation trait

This allows users to write operations for their custom widgets.

native/src/widget/action.rs

@@ -3,6 +3,7 @@ use crate::widget::Id;
 
 use iced_futures::MaybeSend;
 
+use std::any::Any;
 use std::rc::Rc;
 
 /// An operation to be performed on the widget tree.
@@ -84,6 +85,10 @@ where
             ) {
                 self.operation.focusable(state, id);
             }
+
+            fn custom(&mut self, state: &mut dyn Any, id: Option<&Id>) {
+                self.operation.custom(state, id);
+            }
         }
 
         let Self { operation, .. } = self;
@@ -118,6 +123,10 @@ where
         self.operation.text_input(state, id);
     }
 
+    fn custom(&mut self, state: &mut dyn Any, id: Option<&Id>) {
+        self.operation.custom(state, id);
+    }
+
     fn finish(&self) -> operation::Outcome<B> {
         match self.operation.finish() {
             operation::Outcome::None => operation::Outcome::None,

native/src/widget/operation.rs

@@ -9,6 +9,7 @@ pub use text_input::TextInput;
 
 use crate::widget::Id;
 
+use std::any::Any;
 use std::fmt;
 
 /// A piece of logic that can traverse the widget tree of an application in
@@ -33,6 +34,9 @@ pub trait Operation<T> {
     /// Operates on a widget that has text input.
     fn text_input(&mut self, _state: &mut dyn TextInput, _id: Option<&Id>) {}
 
+    /// Operates on a custom widget with some state.
+    fn custom(&mut self, _state: &mut dyn Any, _id: Option<&Id>) {}
+
     /// Finishes the [`Operation`] and returns its [`Outcome`].
     fn finish(&self) -> Outcome<T> {
         Outcome::None