[Feature Request] Let ArrayWidget respect choices parameter

[Vasanth]

When the ArrayField has a CharField with a choices parameter to store select values, it would be great to have a widget that lists the options directly from the available choices.

At the moment, I override an ArrayField with a forms.MultipleChoiceField(widget=forms.CheckboxSelectMultiple, choices=..). This data is then directly stored as an array.

[Lukas Vinclav]

Added #547