Load the setting's cameria preview opengl context lazily and destroy it when done. Only preallocte Core's video buffer if we have any calls active, free up when all calls are done
- Add AvatarBroadcaster, in charge of making sure our friends have our avatar without spamming file transfers
- Fix file sending code not closing the file after transfer, which prevented file previews, and make the QFile a shared_ptr to fix the obvious memory leak
Some small additions to Core to support AvatarBroadcaster
