ENH: Allow application name to be defined by argv0

The qCoreApplication::applicationName controls the discovery of QSettings files and some other parts of the internal machinery of the application. When making a custom version of slicer, we want to keep the settings independent of the original slicer, so that, for example, they can have different module paths related to the extensions that are bundled.

See:

http://www.slicer.org/slicerWiki/index.php/Documentation/Labs/CustomSlicerGenerator