QmlDesigner: Adjust wizard template

Instead of using the constants we can use the size of the
main screen directly. This has the advantage that if
the size of the main screen changes without adjusting
the constants it still works.

Change-Id: I602cf4089249e90efb6a35b803d67df6ca2b1bab
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

share/qtcreator/qmldesigner/studio_templates/projects/common/App.qml.tpl

@@ -35,13 +35,14 @@ import QtQuick.VirtualKeyboard %{QtQuickVersion}
 @endif
 
 Window {
-    width: Constants.width
-    height: Constants.height
+    width: mainScreen.width
+    height: mainScreen.height
 
     visible: true
     title: "%{ProjectName}"
 
     %{UIClassName} {
+        id: mainScreen
     }
 
 @if %{UseVirtualKeyboard}