Instead of having two concurrent display server connections via two
different GdkDisplay's, use a single X11 display connection on X11
sessions, and a single Wayland connection on Wayland sessions.

To make it possible to map Wayland dialogs as parents on top of X11
application windows, use a private X11 interoperability protocol
accessed via a "service channel" Wayland connection, opened via a
special D-Bus interface.

This eradicates the need for forcing the cairo rendererer, as it was
needed due to OpenGL problems handling multiple display server
architectures at the same time. With that obstacle gone, we have no need
to force the cairo renderer anymore.