dropdown - selectAll option handling

[Cardu]

Closes [#2520959](https://buildo.kaiten.io/space/[object Object]/card/2520959)

⚠️ The line above was added during the automatic migration of all github project issues to Kaiten. The old GH issue link is kept here for reference (it is now deprecated, continue follow the original issue on Kaiten).

Closes #947

⚠️ Not ready to merge, just a prototype to show the idea

[Cardu]

Alternative implementation that I was talking about on slack: https://buildo.slack.com/archives/C289TFRAQ/p1568460163000100?thread_ts=1568380065.007700&cid=C289TFRAQ

[Cardu]

Tested customizing the example dropdown like this:

initialState = {};

const onChange = value => value === 'all' ? setState({ value: {value: 'all', label: "tutti quanti"} }) : setState({ value });

const options = [
  { value: "apple", label: "Apple" },
  { value: "avocado", label: "Avocado" },
  { value: "orange", label: "Orange" },
  { value: "lemon", label: "Lemon" },
  { value: "mandarin", label: "Mandarin" }
];

console.debug(state);

<Dropdown
    showSelectAll
    selectAllLabel='selezionami tutto'
  type="single"
  value={state.value}
  onChange={onChange}
  placeholder="Select some fruit"
  options={options}
/>;

[fes300]

@Cardu can we close this?

[Cardu]

@fes300 nope! ⚠️ this branch was just a demonstration and it should be deleted, I think @veej is working on the "good" version of this

[Cardu]

If you meant close without merging and destroy the branch, then definitively yes