Add Disableable trait (#3439)

This PR adds a new `Disableable` trait to use for elements that are
capable of being disabled.

Release Notes:

- N/A

crates/ui2/src/disableable.rs

@@ -0,0 +1,5 @@
+/// A trait for elements that can be disabled.
+pub trait Disableable {
+    /// Sets whether the element is disabled.
+    fn disabled(self, disabled: bool) -> Self;
+}