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/prelude.rs

@@ -1,9 +1,11 @@
+pub use gpui::prelude::*;
 pub use gpui::{
     div, Element, ElementId, InteractiveElement, ParentElement, RenderOnce, SharedString, Styled,
     ViewContext, WindowContext,
 };
 
 pub use crate::clickable::*;
+pub use crate::disableable::*;
 pub use crate::fixed::*;
 pub use crate::selectable::*;
 pub use crate::StyledExt;