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/qml/Frame.qml

@@ -14,13 +14,15 @@ import QtQuick 2.9
 Rectangle {
     id: root
     property QtObject frameCpp
+    readonly property QtObject titleBarCpp: frameCpp ? frameCpp.titleBar : null
 
     color: "cyan"
     anchors.fill: parent
 
     TitleBar {
         height: 30
-        titleBarCpp: frameCpp ? frameCpp.titleBar : null
+        titleBarCpp: root.titleBarCpp
+        visible: true
         anchors {
             top:  parent.top
             left: parent.left