Introduce InitialOption struct

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

src/MainWindowBase.cpp

@@ -97,7 +97,8 @@ void MainWindowBase::addDockWidgetAsTab(DockWidgetBase *widget)
     }
 }
 
-void MainWindowBase::addDockWidget(DockWidgetBase *dw, Location location, DockWidgetBase *relativeTo, AddingOption option)
+void MainWindowBase::addDockWidget(DockWidgetBase *dw, Location location,
+                                   DockWidgetBase *relativeTo, InitialOption option)
 {
     if (dw->options() & DockWidgetBase::Option_NotDockable) {
         qWarning() << Q_FUNC_INFO << "Refusing to dock non-dockable widget" << dw;