Add a new Hooks element to invoke a callback before layout

This is useful to cap the width of sidebars when dragging the
resize handles beyond the maximum bounds of the sidebar.

gpui/src/elements.rs

@@ -5,6 +5,7 @@ mod container;
 mod empty;
 mod event_handler;
 mod flex;
+mod hooks;
 mod label;
 mod line_box;
 mod list;
@@ -23,6 +24,7 @@ pub use container::*;
 pub use empty::*;
 pub use event_handler::*;
 pub use flex::*;
+pub use hooks::*;
 pub use label::*;
 pub use line_box::*;
 pub use list::*;