Replace mixins with other mechanisms

[whitelynx]

Get rid of the entire mixin mechanism except when they actually add functionality to e.g. a Client object. Instead, let each little class that handles some function listen to AddClient? events, and when that happens register event handlers for that new client object (using the mechanism above). Some base class can be written to do the AddClient/RemoveClient? event listening.

[whitelynx]

Mixin audit wiki page started at DesignDocs/MixinAudit.