Add support for album art via avatar setting

[taktoa]

Relevant information from the TeamSpeak forums:

Actually, I think it is possible to set a client avatar with the plugin SDK, but you'll have to do it the manual way which means:

• Upload the avatar file to the virtual server's internal file repository (channel with ID 0) and name it avatar_<client_base64HashClientUID>.
• Modify the client_flag_avatar property for the client by sending a ClientQuery command.
  I've not tested this, but this is how you do it using the ServerQuery interface... so it should work for plugins too.