Create All Default Values With Generators

[rmorshea]

Closes #256

note: all default values (including those defined in trait constructors) are now cross validated

class MyInt(Int):
    def make_dynamic_default(self):
        if self.default_value != 0:
            return self.default_value
        else:
            return 10

class A(HasTraits):
    i = MyInt()

class B(A):
    i = MyInt(20)

class C(B):
    @default('i')
    def _i_default(self):
        return 30

grandparent = super(B, C)
parent = super(C, C)
child = C

c = C()

print(
    grandparent._trait_default_generators['i'](c),
    parent._trait_default_generators['i'](c),
    child._trait_default_generators['i'](c)
)

10, 20, 30

[rmorshea]

Default cross-validation exposes exploits in notebook via #342 as detailed in #315

[SylvainCorlay]

@rmorshea this is the PR that breaks widgets (cf #363 ). Can you try instantiating e.g. an IntSlider ?

[SylvainCorlay]

cc @minrk