Implement Selectable for buttons (#3451)

This PR implements the `Selectable` trait for `ButtonLike`, `Button`,
and `IconButton`.

Release Notes:

- N/A

crates/ui2/src/selectable.rs

@@ -1,15 +1,7 @@
-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>,
-    ) -> Self;
 }
 
 #[derive(Debug, Default, PartialEq, Eq, Hash, Clone, Copy)]