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/customtitlebar/main.qml

@@ -12,9 +12,8 @@
 import QtQuick 2.6
 import com.kdab.dockwidgets 1.0 as KDDW
 
-KDDW.MainWindow {
+KDDW.MainWindowLayout {
     id: root
 
     uniqueName: "MyWindowName-1"
 }
-