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
