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/gpui/src/platform/windows.rs

@@ -11,3 +11,5 @@ pub(crate) use platform::*;
 pub(crate) use text_system::*;
 pub(crate) use util::*;
 pub(crate) use window::*;
+
+pub(crate) use windows::Win32::Foundation::HWND;