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.cpp

@@ -27,6 +27,7 @@
 #include <QApplication>
 #include <QPainter>
 #include <QVBoxLayout>
+#include <QWindowStateChangeEvent>
 
 using namespace KDDockWidgets;
 
@@ -51,6 +52,14 @@ void FloatingWindowWidget::paintEvent(QPaintEvent *)
     p.drawRect(rect().adjusted(0, 0, -1, -1));
 }
 
+bool FloatingWindowWidget::event(QEvent *ev)
+{
+    if (ev->type() == QEvent::WindowStateChange)
+        Q_EMIT windowStateChanged(static_cast<QWindowStateChangeEvent*>(ev));
+
+    return FloatingWindow::event(ev);
+}
+
 void FloatingWindowWidget::init()
 {
     m_vlayout->setSpacing(0);