Maint: Fix correctness in Enum default traitsui editor

[kitchoi]

Closes #965

This PR:

• changes value = self (self is an instance of Enum) to value = self.values. Both works, but the latter is more correct.
• Workaround format_func not used when EnumEditor is initialized with static values traitsui#782 when the allowed values are defined statically and when the format_func is provided. I'd not recommend putting format_func in the definition of trait, but it needs to stay for backward compatibility.

For example, this:

from traits.api import HasTraits, Enum

class Foo(HasTraits):

    true_or_false = Enum([0, 1], format_func=lambda v: str(bool(v)).upper())

foo = Foo()
foo.configure_traits()

We'd expect to see the dropdown contains "TRUE" and "FALSE". enthought/traitsui#782 causes us to see "0" and "1" instead.

Checklist

• Tests
  Not testing the effect of format_func here to reduce test dependencies on traitsui. See previous discussion in Add a GUI test for Enum.create_editor #988. See Move default traitsui editors over to traitsui #1004 as well.
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in traits-stubs

[ievacerny]

The code looks good and I can verify that Enum dropdown now shows "FALSE" and "TRUE" instead of "0" and "1" 👍

[kitchoi]

Thanks!