Feature/disabled checkbox

[zgr024]

Adds the ability to disable the checkbox for an individual node

const treeState = {
  name: 'root [half checked and opened]',
  checked: 0.5,   // half check: some children are checked
  isOpen: true,   // this folder is opened, we can see it's children
  children: [
    { name: 'children 1 [not checked]', checked: 0 },
    {
      name: 'children 2 [half checked and not opened]',
      checked: 0.5,
      isOpen: false,
      children: [
        { name: 'children 2-1 [not checked]', checked: 0 },
        { name: 'children 2-2 [checked]', checked: 1 },
        { name: 'children 2-3 [not checked] [disabled]', checked: 0, isDisabled: true },
      ],
    },
  ],
};