Add native toolbar GPUI overlay anchors

[naaiyy]

Summary

• add GPUI support for hosting GPUI-rendered visuals inside native toolbar buttons and menu buttons
• expose native toolbar item bounds in window coordinates for pure GPUI overlays anchored from native toolbar items
• add example coverage for hosted toolbar content and pure GPUI overlay menus from native toolbar triggers

Release Notes:

• Improved native toolbar integration so GPUI content and overlays can be anchored from native toolbar items on macOS