set up entry point libs in Qt.core

Let the Qt profile setup write the entry point libs to Qt's core.qbs.
Also, add shell32.lib to MSVC if we have a Qt >= 5.4.

Change-Id: I8ade1141101ac7494031ad49e78ba63be0342f03
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>