Crash when holding 'next frame'-button down too long

When holding the next-frame-button for round-about 60 frames, PeTrack crashes. This is due to a stackoverlow.

In Player::updateImage there is an call to qApp->processEvents() which results in Player::forward being called, which results in Player::updateImage being called. Closed curcuit...

The call to qApp->processEvents does not seem to be important on my machine, but such a method on ensuring responsibility can vary wildly with hardware of different performance. So yeah, I'd remove that one for now, but we'll need to make the responsive design more independent from calls to processEvents.