addDockWidget() received a AddingOption enumerator. But that enum isn't enough, as we also want to be able to specify an initial QSize. So, instead of adding yet another parameter to addDockWidget(), we create a struct to old those "initial settings". Implies a minor source break, but trivial to fix in user code, just rename the enumerator. Honouring preferredSize will be a followup. Needed for issue #95