Remove thing_changed Event traits if possible

**Problem Description**
I personally find it very confusing to have a trait named `thing_changed = Event` which explicitly gets set to `True` when `thing` changes. Chaco will often listen to that trait when setting up change handlers, but I am not seeing why we couldn't just listen to the trait itself. e.g why do something like `observe(_____, thing_changed)` instead of just `observe(_____, thing)`.
I may be missing something important as to why this is needed but as far as I can tell it is not.

See #601 which removed the `metadata_changed` event trait. Other examples include:
https://github.com/enthought/chaco/blob/ef7227706642f7d26042717ad9b60648d7572068/chaco/base_2d_plot.py#L63-L76
https://github.com/enthought/chaco/blob/ef7227706642f7d26042717ad9b60648d7572068/chaco/base_2d_plot.py#L278-L284
https://github.com/enthought/chaco/blob/ef7227706642f7d26042717ad9b60648d7572068/chaco/base_2d_plot.py#L313-L332

In any case, if there is a reason that something like this needs to be done, we should still make an effort to clean this code up and make it more easily understandable / better documented of why things are like this.