under gnome-shell which doesn't put an "X" to close things in
the wm decorations if the GDK DIALOG hint it set

prior to tdf#72587 this was a modeless dialog and in that case
it doesn't have a parent, in which case the hint is NORMAL
and it used to get close decorations.

easist thing to do is to remain modal so the crashes and
confusion of tdf#72587 remain fixed but add a close to it to
behave like all the other dialogs do

Change-Id: I64450be0ad1bc7b06196e1342679e15b60fc60d9