Refactor listeners which are called standalone

[aaronayres35]

ref: #864

Previously with on_trait_change handlers could be defined without arguments and occasionally code would call these handlers independent of any trait listening. With observe handlers are required to take a single event argument. So any such standalone callers are now passing event=None.
What we should do instead is, if any handler is such that we want to be able to call it stand alone, we should make it a regular old method that we call stand alone, and then have the handler call that method.

[aaronayres35]

duplicate of #876
Forgot I had already opened an issue