See 747a0fdda2
("SaveToolbarController: Better support of readonly docs")
for the background. However this doesn't work, because
the save slot is disabled in other cases too, not just for
r/o docs (e.g. locked UI during file save, or when cond.
formatting window open in Calc).
Until a better solution is found, at least make sure that
the button is disabled, instead of going into the normal
save mode, which doesn't work.
Change-Id: If6e1b91b7c780fffac22cdb25c82bbd0a8520dc4
011128aa94