Fix PlayerManager::mainPlayer uninitialized

SoundMain may call getMainPlayer before the creation of the player.
Fix/allow this by initializing to mainPlayer nullptr.
Fix SoundMain deleted too early during shutdown

BackgroundMusicHandler has a unique_ptr to a MusicSource whose
destructor needs SoundMain, but SoundMain was deleted before
BackgroundMusicHandler.

This patch explicitely deletes BackgroundMusicHandler first.
Use fixed camera to fix cam stutter/jitter bug

We don't update the camera's position during the frame events anymore,
it's done automatically & immediatly when the kart moves. This fixes the
camera stutter.

But the camera is just a fixed node relative to the kart, it looks very
simple.
Merge branch 'dev' of https://github.com/Ponykart/PonyKart into dev