nested mdi: Fix floating picking the wrong title bar

Use DockWidget::titleBar() which will always travel the hierarchy
and pick the first visible title bar. We have more nesting now.

src/DockWidgetBase.cpp

@@ -205,7 +205,7 @@ bool DockWidgetBase::setFloating(bool floats)
 
             frame->detachTab(this);
         } else {
-            d->frame()->titleBar()->makeWindow();
+            titleBar()->makeWindow();
         }
 
         auto lastGeo = d->lastPositions().lastFloatingGeometry();