macOS: add ability to make titlebar unified

[valentinegb]

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

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Adds WindowExtMacOS::set_unified_titlebar and WindowAttributesExtMacOS::with_unified_titlebar, which allow you to use a larger titlebar style on macOS.

Without unified titlebar (default)	With unified titlebar

The titlebar can be made transparent and the title can be hidden and there will be extra space around the window controls.

My code is probably not perfect, I would gladly accept edits or suggestions!