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
• Cat
• Human

Reptiles

• Snake
• Mark Zuckerberg
• Turtle
  etc.

[SpaceK33z]

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