MDL-34728 forms: woy for other JS to trigger disableIf update.
This was discovered while working on MDL-32705. If some JavaScript (for
example a select all/none link) changes the state of some form fields,
then the disabledIf state of other form elements does not automatically
update.
The existing form JS was so well encapsulated that this was impossible.
This change pokes a hole in the encapsulation, and provides an API
M.form.updateFormState(formid);
that other bits of JS code can call when necessary.
