X11Interop is not usable with Wayland

[philstopford]

There needs to be a way of detecting Wayland and using its interop instead. I played around a bit, but I haven't been able to find a way to detect whether the display is X11 or Wayland and GTK3 uses a macro, which makes it more awkward still:

#define GDK_IS_X11_DISPLAY(object)        (G_TYPE_CHECK_INSTANCE_TYPE ((object), GDK_TYPE_X11_DISPLAY))
#define GDK_IS_X11_DISPLAY_CLASS(klass)   (G_TYPE_CHECK_CLASS_TYPE ((klass), GDK_TYPE_X11_DISPLAY))