Let user create draggable control points

Whenever we make the change we discussed where we go from

```javascript
{..., _selected: '...', _layout: {...}, ...
```
 to

```javascript
{..., _meta: {selected: '...', layout: {...}}`
```

i think it would make more sense to move `colour` directly into that `_meta` object, since its not strictly related to layout (similarly to `_selected`).