This works the same as the customclassoverride on the submit element,
in that only the 'btn' class is hard coded in the template. Passing
$options['customclassoverride'] to the constructor allows calling code
to specify other classes to be added beside 'btn'. E.g. 'btn-primary'.
If no options array is passed, 'btn-secondary' is used (the existing
class used for button elements).
In PHP 8.2 and later, setting a value to an undeclared class property is
deprecated and emits a deprecation notice.
So we need to add missing class properties that still need to be declared.
This change allows form elements to be overridden with a mustache template.
The template can even listen for form validation errors and supply the JS to
change the look of the form element when there is/isn't a validation error.
Initial support is for all core form elements including:
text, select, selectyesno and checkboxes, groups, dateselector, datetimeselector,
autocomplete, modvisible, advcheckbox, button, duration, filemanager, filepicker, editor, static, grading,
warning, textarea, password, url, submit, questioncategory, recaptcha.
Part of MDL-55071
Jake Dallimore