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

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24004 3bd1e089-480b-0410-8dfb-8563597acbee