Make sizing of ComponentEditor easier

A common issue encountered by users of Enable and Chaco is how to handle the sizing of the main `ComponentEditor` that contains the plot or other drawing being displayed in a TraitsUI. Currently there are a number of places that size can potentially be controlled:

- the `height` and `width` of the TraitsUI `Item` and it's `springy` and `resizable` traits (plus potentially other factors like the size and scrollability of other items and the view itself).
- the `size` supplied to the `ComponentEditor` itself
- the `position` and `bounds` of the top-level `Component` and whether or not it resizes

Some of these have an effect, some do not, or only do in the absence of other preferences being supplied.

There is also the consideration of what happens when a UI is resized by a user and how to control the growth of the component.

At a minimum these behaviours should be documented. Even better is to think through the design to make it clear what controls what and how, and provide simple and clear direction about what to set and what the effect will be.