adds a base class for mixin or composed classes managed by agents

A number of toolkit classes had identical interfaces to reach the managing agent. This PR gives a base mixin class that provides the common interface. It can be used for future derived mixins or composition classes.

I'll note that this base class could be moved up into the kernel in the future if we desire (`Trader`s have very similar functionality).

I also took the opportunity to fix the include guards for toolkit classes.