Finish FocusScope support

When clicking on a TitleBar the focus will be redirected to either:
- Last widget that had focus inside the scope
- To the DockWidget. Implies the user setting a guest widget that
accepts focus

Fixes: #56

examples/dockwidgets/MyWidget.cpp

@@ -41,6 +41,7 @@ MyWidget::MyWidget(const QString &backgroundFile, const QString &logoFile, QWidg
     new QLineEdit(this);
     auto l2 = new QLineEdit(this);
     l2->move(0, 100);
+    setFocusProxy(l2);
 #endif
 }