Just to keep it hidden so far... A lot of the plumbing necessary to
handle x11->wayland drag and drop is missing, and the current
partial handling gets in the middle for X11 drag-and-drop itself
to work.

The approach is well directed, but needs some further work, till
then, just keep our fake drag-and-drop target hidden. This allows
drag-and-drop to work between X11 clients in Xwayland, and avoids
a crash with (currently unhandled) wl_resource-less data sources.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=94218



Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
Reviewed-by: Daniel Stone <daniels@collabora.com>