BUG: Remove forgotten Qt style

The style Macintosh (aqua) was still available on MacOS in the settings
even though it's a default Qt style.
Adding it to the list of Qt default style removes it.
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 by slicer before the
qSlicerApplication is created.

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