SortableList should support separator items

[ythomop]

Hello all.

A common use of sortable lists is to let the users move items between two or more lists. Usually, such lists are indicated by separator elements. An example of the intended functionality can be found here. This was made using Ionic which and the source can be found here.

Until now, doing this with RNUILib is impossible, except for the case where I'm missing something.

Anyway, I have forked and have made the relevant changes. (See here) They are just 3 lines. With these changes, the example I linked above can be accomplished with this data array:

const data: SortableListItemProps[] = [
    {
      id: 'list1',
      separator: true
    },
    {
      id: 'item1'
    },
    {
      id: 'item2'
    },
    {
      id: 'item3'
    },
    {
      id: 'list2',
      separator: true
    },
    {
      id: 'item4'
    },
    {
      id: 'item5'
    }
  ];

Almost always, the first separator must be locked at the top. With this implementation, both locked and separator fields can be set to true to provide this functionality.

Is it worth making the PR or is this feature irrelevant to RNUILib?