BUG: Use qSlicerStyle on start-up no matter what

When the launcher isn't used, for example when using SlicerApp-Real
directly, the plugin library cannot be found before the qSlicerApplication
is created. (QT_PLUGIN_PATH isn't set).

However once the application is started, the current location of the
executable is automaticaly added to the application library paths by Qt.
See Qt-doc: http://qt-project.org/doc/qt-4.7/qcoreapplication.html#id-a4ff2599-e40c-4218-82e7-14e570ebf822
Since this also corresponds to the location that needs to be set for the
plugins, the styles are automaticaly discovered.

Fixes #3252