wayland: Add X11/wayland selection interoperation

This piece of code hooks in both wl_data_device and the relevant X
selection events, an X11 Window is set up so it can act as the clipboard
owner when any wayland client owns the selection, reacting to
SelectionRequest events, and returning the data from the wayland client
FD to any X11 requestor through X properties.

In the opposite direction, SelectionNotify messages are received,
which results in the property contents being converted then written
into the wayland requestor's FD.

This code also takes care of the handling incremental transfers through
the INCR property type, reading/writing data chunk by chunk.

https://bugzilla.gnome.org/show_bug.cgi?id=738312