Cross-platform titlebar (#9405)

This PR reverts https://github.com/zed-industries/zed/pull/9392 and
fixes the regressions that led to the reversion.

Release Notes:

- N/A

---------

Co-authored-by: Ezekiel Warren <ezekiel@seaube.com>

crates/ui/src/components.rs

@@ -9,6 +9,7 @@ mod indicator;
 mod keybinding;
 mod label;
 mod list;
+mod platform_titlebar;
 mod popover;
 mod popover_menu;
 mod right_click_menu;
@@ -31,6 +32,7 @@ pub use indicator::*;
 pub use keybinding::*;
 pub use label::*;
 pub use list::*;
+pub use platform_titlebar::*;
 pub use popover::*;
 pub use popover_menu::*;
 pub use right_click_menu::*;