Add a separate GTK-specific wxDisplay implementation.
This separates the GTK implementation from Unix/X11, except for the wxVideoMode
stuff, and _NET_WORKAREA when GTK < 3.4. wxDisplay and wxClientDisplayRect()
should now work as well as they can with Wayland.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76365 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775