Simplify usage of QPainter::save/restore() for styles

It would be nice to have a small RAII helper class which makes sure that QPainter::restore() is called for every QPainter::save(). Or since QPainter::save()/restore() is a heavy operation, a helper class which only saves the states which are getting modified.

The first user should be the internal Qt styles. When it becomes mature, it can become a public class for wider usage.