subtype value silently dropped when saving formBuilder when subtype field is not available

[lucasnetau]

          I just figured that once you saved the form and loaded again, the fields get lost from the ui. That's because the checkbox group does not have the Type dropdown with the subtype. I'm looking to see if that's possible to add it, that way the declared subtype is still in the element once saved/loaded.

Originally posted by @parrainc in #1168 (comment)

A checkbox-group/radio can be added with a subtype defined. This allows subtype userAttributes to be applied:

{
      type: 'checkbox-group',
      subtype: 'custom-group',
      label: 'Custom Checkbox Group w/Sub Type',
      required: true,
      values: [
        { label: 'Option 1' },
        { label: 'Option 2' }
      ]
}

However because this control does not show a subtype attribute in the stage (guess is because controls.registeredSubtypes does not return subtypes defined in opts.subtypes) the value is lost when saving the formdata

{
      type: 'checkbox-group',
      label: 'Custom Checkbox Group w/Sub Type',
      required: true,
      values: [
        { label: 'Option 1' },
        { label: 'Option 2' }
      ]
}

Thoughts so far:

• Use config.subtypes instead of controls.registeredSubtypes to decide on showing the subtype dropdown
• For control select.js need to ensure that when building we use the type and not subtype for rendering due to this code in control.js

    // if subtype, update the config type for injecting into DOM elements
    if (this.subtype) {
      config.type = this.subtype
    }