feat(Dropdown): support label prop

[levithomason]

UPDATE

This will likely be solved by augmentation, see #414.

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A Label in a labeled Input can actually be a dropdown, see here. This is not the same thing as a labeled dropdown, which separates a dropdown button icon from the button text.

Dropdown Label

It appears the only necessary extra dropdown class required is label. I'm not entirely sure how we want to handle this yet. We could simply add a label prop that adds this class.

I do not think we want to automatically add this when used inside an Input though. A dropdown can also appear as an Input action:

Dropdown Button

Dropdown

[levithomason]

Solved. The Input and the Button will simply add the className when creating the Dropdown via their factories:

const labelElement = createLabel(label, { className: 'label' })