Dynamic Defaults Using New Decorator API

[rmorshea]

brief example

class A(HasTraits):
    i = Int()

    @default('x')
    def _x_default(self):
        return 1

A().i

result : 1

[rmorshea]

builds on #73 to expand the decorator api and replace legacy magic methods

[rmorshea]

@SylvainCorlay, @jasongrout this pr is ready for review.

[SylvainCorlay]

I guess that you will want the same mechanism as for the validate handler to avoid the registration of two default handlers and handle the case where the old method is registered with the decorator.

I think we should probably wait for the other PR to be merged before getting more into this one.

[ellisonbg]

The setting and getting logic in this class needs to be audited @SylvainCorlay

[SylvainCorlay]

This needs a rebase.

[SylvainCorlay]

Since there are already _trait_notifiers, _trait_validators and _trait_values, this could be made private and called _trait_default_generators or something like this. Never mind.
Why don't you register default handlers at the HasTraits instance level?

[rmorshea]

@SylvainCorlay removed the need for storage of EventHandlers on TraitType instances and allowed multiple trait names in the default decorator as in observe and validate.

[rmorshea]

@SylvainCorlay new dynamic default decorator logic.