list of groups in the collapse widget. Each array element represents a single
group with the following structure:
- label: string, required, the group header label.
- encode: boolean, optional, whether this label should be HTML-encoded. This param will override
global $this->encodeLabels param.
- content: array|string|object, required, the content (HTML) of the group
- options: array, optional, the HTML attributes of the group
- contentOptions: optional, the HTML attributes of the group's content