qml: Layout the drop area and title bar inside the FloatingWindow

We could just control geometry of the c++ title bar and geometry,
but then the user wouldn't be able to style it, and users rather
style things in QML than subclassing FloatingWindow and writing C++.

So instance some .qml files to represent the drop area and floating
window, so user can muck with.

Hierarchy looks like this now:

-QQuickWindow
--FloatingWindowQuick (C++ QQuickItem)
---FloatingWindow.qml
----TitleBar.qml (reads state from TitleBar c++)
----DropArea.qml
-----DropArea (C++ QQuickItem)
------Frame
      etc

src/private/quick/QWidgetAdapter_quick.cpp

@@ -24,6 +24,9 @@
 #include <QResizeEvent>
 #include <QMouseEvent>
 #include <QWindow>
+#include <QQmlComponent>
+#include <QQuickItem>
+#include <QQmlEngine>
 
 using namespace KDDockWidgets;
 
@@ -211,6 +214,18 @@ Qt::WindowFlags QWidgetAdapter::windowFlags() const
     return m_requestedWindowFlags;
 }
 
+QQuickItem *QWidgetAdapter::createItem(QQmlEngine *engine, const QString &filename) const
+{
+    QQmlComponent component(engine, filename);
+    QObject *obj = component.create();
+    if (!obj) {
+        qWarning() << Q_FUNC_INFO << component.errorString();
+        return nullptr;
+    }
+
+    return qobject_cast<QQuickItem*>(obj);
+}
+
 void QWidgetAdapter::setFlag(Qt::WindowType f, bool on)
 {
     if (QWindow *w = windowHandle()) {