Form::createElements() should add elements instead of returning them

In case createElements() would still return the elements while requiring
the caller to add them to the form all form dependent configurations get
lost. (displaygroups, belongTo, ...) Wizards or parent forms can still
retrieve only input relevant fields by just calling createElements() and
getElements().

refs #5525
Merge branch 'bugfix/rebuild-form-builder-5525' into bugfix/commands-6593