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/clickable.rs

@@ -1,5 +1,7 @@
 use gpui::{ClickEvent, WindowContext};
 
+/// A trait for elements that can be clicked.
 pub trait Clickable {
+    /// Sets the click handler that will fire whenever the element is clicked.
     fn on_click(self, handler: impl Fn(&ClickEvent, &mut WindowContext) + 'static) -> Self;
 }