tdf#150278: avoid gen/gtk3 difference in m_xPropBox->GetCurPage()
When switching between tabs, at this call SAL_USE_VCLPLUGIN=gen
was returning the switched-to-tab id, while gtk3 was still
returning the leaving-tab's id.
So just ignore all that an use the OnActivatePage value.
Change-Id: I166626eac033af65e95f8a280076a6bcc070bee3
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/138003
Tested-by: Jenkins
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>