Make Clipboard argument in Widget trait mutable

native/src/clipboard.rs

@@ -1,3 +1,5 @@
+//! Access the clipboard.
+
 /// A buffer for short-term storage and transfer within and between
 /// applications.
 pub trait Clipboard {
@@ -7,3 +9,15 @@ pub trait Clipboard {
     /// Writes the given text contents to the [`Clipboard`].
     fn write(&mut self, contents: String);
 }
+
+/// A null implementation of the [`Clipboard`] trait.
+#[derive(Debug, Clone, Copy)]
+pub struct Null;
+
+impl Clipboard for Null {
+    fn read(&self) -> Option<String> {
+        None
+    }
+
+    fn write(&mut self, _contents: String) {}
+}