Add getValue/setValue/hasValue and isValueEmpty to FormState

Problem/Motivation

FormState should provide methods to interact with $form_state['values'].

Proposed resolution

Add get/set/has methods.
Instead of just supporting a scalar key, this should also allow arrays to be passed in, to function like #parents does.

// D7
if (isset($form_state['values']['foo']['bar'])) {
}

// D8
if ($form_state->hasValue(array('foo', 'bar'))) {
}

Remaining tasks

User interface changes

API changes