qtquick: the user will now use "MainWindowLayout" instead of "MainWindow"

As MainWindow wasn't actually a window.

While for QtWidgets it makes sense for it to be a QMainWindow, in
QtQuick let's make it easy to embed a dock widget layout.

Also, our MainWindowQuick doesn't have any menus and what not that
QMainWindow has.

examples/qtquick/mdi/main.qml

@@ -13,7 +13,7 @@ import QtQuick 2.6
 import com.kdab.dockwidgets 1.0 as KDDW
 import "qrc:/kddockwidgets/private/quick/qml/" // TODO: Improve the public API
 
-MainWindowMDI {
+MainWindowLayout {
     id: root
     uniqueName: "MyWindow1"
 }