Improve TabBar return types

DeactivatePage() goes from returning long to returning bool.
StartRenaming goes from returning long to returning bool.
AllowRenaming goes from returning long to returning a
constant from a new enum.

Change-Id: Ib87e8d7ceb4e23846547257b358ca4dfd41eb6ca
Reviewed-on: https://gerrit.libreoffice.org/9161
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>