Allow creation via class + attributes

[twidi]

Something like

class States(Choices):
    ONLINE = (1, 'Online')
    DRAFT = (2, 'Draft')
    OFFLINE = (3, 'Offline')

An attribute could accept a tuple, as above, or a single value, in which case we have to decide if the display would be the attribute name or the value.
The right part may even be directly a ChoiceEntry.

[twidi]

Yes, with python 3 it could be an enum. But not sure I can do everything that is already done in the lib with enums. To check but after v2 (see #35 for that)