Move FormHelper::_domId() to IdGeneratorTrait.
Add ID attribute prefixing feature.
Implement id prefixing for radio and multicheckbox widgets.
Remove extra newline.
Move View/Widget/IdGeneratorTrait to View/Helper.
Merge pull request #2980 from ADmad/3.0-form-idprefix

3.0 form idprefix

Fixes #2952