a11y: Add a display to GtkATContext

a11y: Add a display to GtkATContext

Since we need to check at run time what kind of AT context to use, we
need a hook into the whole GDK backend machinery. The display connection
seems to be the best choice, in this case, as it allows us to determine
whether we're running on an X11 or Wayland system, and thus whether we
should create a GtkAtSpiContext.

This requires some surgery to fix the GtkATContext creation function, in
order to include a GdkDisplay instance.