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/selectable.rs

@@ -1,7 +1,11 @@
 use gpui::{AnyView, WindowContext};
 
+/// A trait for elements that can be selected.
 pub trait Selectable {
+    /// Sets whether the element is selected.
     fn selected(self, selected: bool) -> Self;
+
+    /// Sets the tooltip that should be shown when the element is selected.
     fn selected_tooltip(
         self,
         tooltip: Box<dyn Fn(&mut WindowContext) -> AnyView + 'static>,