Add and make use of convenience item QtGuiApplication.

Using this item, building a Qt gui app against a static Qt
will work out of the box.
Also add the less useful QtApplication for completeness.

Change-Id: Idca1dbd3085659ebaf563f7df89f73b9daaf7001
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>