Remove a dock widget from the layout before adding as tab

So the layout can grow first and not complain there's not enough
space.

Fixes fuzzer test-case 3.json.
The warning was: "Constraints not honoured" in Item::onLayoutRequest()

src/DockWidgetBase.cpp

@@ -154,6 +154,7 @@ void DockWidgetBase::addDockWidgetAsTab(DockWidgetBase *other)
     }
 
     Q_ASSERT(frame);
+    other->setParent(nullptr);
     frame->addWidget(other);
 }