Support insetting the macOS window buttons

[nicoburns]

Description

Apps like Chrome, Firefox and VS Code add custom insets to the macOS window buttons. It would be useful to be able to do this in Winit apps.

• Inset Mac OS traffic lights tauri-apps/tao#513 implements this in Tao
• https://stackoverflow.com/questions/30085631/nswindow-change-positions-of-window-buttons describes a general approach using Swift

Relevant platforms

macOS