Introduce "Maximize/restore" title bar button support

As happens with other docking frameworks:
- Maximized/restore button only visible when window is floating
- floating button is never visible

Available by setting Flag_TitleBarHasMaximizeButton.
Added example too.

Fixes: #37

src/private/widgets/FloatingWindowWidget_p.h

@@ -38,6 +38,7 @@ public:
 
 protected:
     void paintEvent(QPaintEvent *) override;
+    bool event(QEvent *ev) override;
 private:
     void init();
     Q_DISABLE_COPY(FloatingWindowWidget)