qt: get rid of QVLCApp

I do not see a valid reason for the existence of this class.

Why?

• Qt favors aggregation, usually. Especially with such specialized classes.
• We call static functions using QApplication in the code base rather than QVLCApp. If this merge request is not approved, we should change these to QVLCApp.
• QVLCApp has been present before 2010, and nothing much added to it or changed.
• No need to add a new macro based on qApp (additional casting required).