Fix to commit 03568b2e2405fc44435b95720f3680ab1daea836

During a rebase I accidentally removed the VirtualDevice::EnableRTL().
Also taking the opportunity to make the function virtual in window.hxx.

Change-Id: Ic239d8dc131dfcc6b049c30d2fad4d2d12059059
Reviewed-on: https://gerrit.libreoffice.org/8745
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>