make run virtual and fold executes into it, so GenericUnoDialog
can call run on tabdialogs to do the right thing, and allows
Start_Impl to be private again

Change-Id: Ic457edfbdc7457f4c49d4e8ad679903f38ad9b42
Reviewed-on: https://gerrit.libreoffice.org/62227
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>