Currently, unprotecting a sheet clears the protection options it
had (i.e. if to allow users to select protected/unprotected cells).
So, protecting next time requires setting these options again.
Unprotecting a sheet usually happens when author edits e.g. a form
and intents to re-protect it when the edit is done, so it would be
handy to restore previous flags as they were before.

This patch allows saving the protection settings by using
current ScTableProtection copy with its protection set to appropriate
value, instead of new clean ScTableProtection or nullptr.

This does not allow to save the flags between edit sessions if the
protection is unset.

Change-Id: Ieaba4eee746efcf05308dbc88e402c200a59b0d2
Reviewed-on: https://gerrit.libreoffice.org/31044Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>