Add QtTools' include/QtDesigner as well as QtCore's include/QtDesigner

Add QtTools' include/QtDesigner as well as QtCore's include/QtDesigner

In modularization QT+=uilib adds the QtCore specific include/QtDesigner,
while QT += designer adds the QtTools specific one.

Using !isEmpty(QT.<module/library>.name) is the proper way to check for
module/library existance in modularized Qt.

Change-Id: If1fe5d192129fd1cdbf43183b52327d7fa9c57ec
Reviewed-on: http://codereview.qt.nokia.com/126
Reviewed-by: Oliver Wolff <oliver.wolff@nokia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>