Support simple conditional logic

[sergei-maertens]

Formio supports simple conditional logic in the form of:

{
  "conditional": {
    "show": true,
    "when": "someOtherComponent",
    "eq": "someValue"
  }
}

which changes the visibility of a component depending on the value of the other component it references.

Notable challenges to take into account:

• Ensure that this works with/within layout components like fieldset and columns
• Implement the clearOnHide behaviour for components that get hidden. Take into account components that are hidden because their parent is hidden.
• Ensure the behaviour in editgrid is consistent with the Formio reference (there are some odd mechanisms to refer to components in/outside of the editgrid)