Deprecate the existing domId method

given the limited places that call domId - keep as is and just
deprecated it. The core won't be using it, but user-land code might
move domId logic from the abstract class down

since it's just the form helper that uses it. Also given how domId is
used - simplify usage. It's not necessary to keep the existing calls to
domId, we can just use the newer methods
Ensure checkboxes and selects have a unique id

If there's a collision with something the form helper already knows
about - it'll increment a counter on the end of the dom IDs