Add groupings of items to dropdown-selects

[Rutger]

It would be nice if dropdowns like FancySelect, MultiPick and MultiSelect would have groupings/heading.

The dropdown of a MultiSelect could look a bit like this:

Mammals

• [ ] Doggo
• [x] Cat
• [ ] Human

Reptiles

• [x] Snake
• [ ] Mark Zuckerberg
• [ ] Turtle etc.

[SpaceK33z]

Note that the TypeAhead component already has this, so you can copy it from there.