Fix crash on shutdown caused by duplicate delete on a QObject

This was a lesson in how to use ref-counted smart pointers.  I guess we all need one of these every once in a while.

The MyAvatar instance now lives in AvatarManager.  The Application class has a (non-smart) pointer to it but doesn't own it.  Someday the thing to do is to cure Application from its need to hang onto a persistent pointer to a MyAvatar -- it should request it from an AvatarManger instance when it needs it.