Provides a base class for form element plugins.
Form elements are a subset of render elements, representing elements for HTML forms, which can be referenced in form arrays. See the Render API topic for an overview of render arrays and render elements, and the Form API topic for an overview of forms and form arrays.
The elements of form arrays are divided up into properties (whose keys start with #) and children (whose keys do not start with #). The properties provide data or settings that are u