Fix sound effects not loading properly. Downmix sound effects to mono.
Properly set audio listener orientation.
Fix float NaN bug, compilation on Windows

Also sets Applejack's cart as the default again, since it's less prone
to falling on its back and getting stuck. Until we have the self-righter
that's probably the best default.
Fix listener orientation reset.
Add more exception catching/loging
Minor change, /* */ to // in .muffin file

Because you can't nest /* */ style comments and it makes playing with
the files somewhat slower.
Remove the CHECKER.DDS box of Level0

The same box that is never used but appears in the middle of Sweet Apple
Acres because we load Level0 with the GUI, but level unloading isn't
implemented so we never unload it.

We don't need it anyway, so bug "fixed".
Add BackgroundMusicHandler
Add LOOPSTART tag to sweet apple acres music.
Move plugins.cfg to user folder and generate it if it doesn't exist.
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.