Add nextByDirection and refactor

[borisbelmar]

• Add nextByDirection to elements, now accepts groups too

api.registerElement(element, 'group-1', {
  nextByDirection: { // This will set the next element manually
    'down': 'element-0-1', // The next element when the user press the down arrow key
    'right': { id: 'group-1', kind: 'group' }, // The next group when the user press the right arrow key
    'up': null, // If press up, no elements will be focused
    'left': undefined // undefined will keep the default behavior
  },
  onFocus: ({ current, prev, direction }) => console.log(`focused ${current.id}`), // Callback when the element is focused. The prev element is the element that was focused before the current element.
  onBlur: ({ current, next, direction }) => console.log(`blurred ${current.id}`) // Callback when the element is blurred. The next element is the element that will be focused when the focus leave the current element.
})

• Add id to elements, thinking about a future react native adapter on the core
• Group elements now is a set of ids that references elements on main Map in the state.
• Deprecated nextElementByDirection