This is needed, as e.g. sd::FuText::DoExecute() queries the mouse
position from VCL, which won't be correct in case the mouse event is
generated by a LOK client, not by VCL. So add a method that makes it
possible to set the position before MouseButton*() starts.

Change-Id: I1e24ffb2a4a866c9cac89367907ffaec336ad1f6