Add missing win32/mac/broadway specific APIs

[bilelmoussaoui]

GDK has specific code like the wayland/x11 ones but for macos/win32 & broadway. We should figure out how to handle those.

Definitely not an urgent thing and before we add any win32/macos code, we should figure out the CI first.

The code would live in the gdk crate directly, there's no need to create different crates unless gtk figures out the introspection data story.

• [x] win32
• [ ] broadway
• [ ] macos