wayland/gtk: Only perform allowed titlebar gestures

The window functions "work" regardless of whether the client allows
the behavior or not. That is, it's up to the caller to not call
maximize and friends when the action isn't allowed.

Add appropriate checks, which should make the titlebar_gesture()
behavior identical to titlebar actions for server-side decorations.

https://gitlab.gnome.org/GNOME/mutter/-/issues/2139

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2281>