Abstract base classes should inherit from ABCHasTraits

[corranwebster]

There are a number of abstract base classes in the Chaco codebase, but the abstractness is not enforced. This leads to problems with partial implementations of the abstract base class interface in subclasses.

All abstract classes should inherit from ABCHasTraits which ensures that:

• the abstract base class cannot be instantiated
• all subclasses must at least provide a minimal implementation of the interface

This would be a backwards incompatible change, as user code which inherits from abstract base classes but implements the interface incompletely may break.

[jonathanrocher]

Thanks. We could consider starting a list of backwards incompatible changes for Chaco 5.0 release. Will tag this issue to that.