Use Traits observers instead of listeners

[corranwebster]

This perhaps should wait for Enable 5.1 rather than 5.0, but we should switch to using the new Traits observer system rather than the current listener system (ie. on_trait_change).

[aaronayres35]

NOTE:
Some of the changes in enable will require a corresponding change in traitsUI. For example trying to observe a method defined on a base class in traitsUI in enable code. That handler method will need its signature changed for things to work.
ref: #663 (comment)