[macos] Use system shadow on macOS
This MR is building on !5194.
Client side shadows do not work really nice for macOS (and Windows?).
This MR allows for CSD without client side shadows. This allows us to have a much more natural integration with macOS.
Exposé view before and after (note that libadwaita's shadow is a lot wider than GTK's default shadow):
Fixes #6255 (closed).
@paulrouget @lb90 @otte @chergert I created this MR, hope that's okay...