so a SfxTabPage can be parented by a vcl::Window or a welded native notebook tabpage.

That ways the same SfxTabPage can be used at the same time in both a native dialog
or a vcl dialog. The impl can be changed to the weld api, and when hosted in a native
dialog the vcl impl of that will be instantiated, while native otherwise. e.g.
print options appearing in print options dialog and general options.

This allows incremental changeover.

Change-Id: I6f1fed1e8d0898b01853bb878757bad41cbf9bba
Reviewed-on: https://gerrit.libreoffice.org/53193Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>