#ief_element_submit callbacks are attached more than once

The code that triggers #ief_element_submit callbacks is attached once in hook_form_alter() to cover IEF widgets, and once in the InlineEntityForm #process, to cover IEF form elements. This results in the code being run once for the widgets, and once for each IEF form element. That's why dawehner had to add array_unique() to the attach code, the callbacks were getting duplicated.

My initial idea was to make sure ElementSubmit::attach is called from only one place, hook_form_alter(). Of course, the form alter doesn't detect IEF form elements because they're created in #process. The only callback that runs after #process is #after_build, but at that point it's too late to add submit callbacks.

So, a few hours later, back to the drawing board. We keep hook_form_alter() as it is, clean up the attaching in the form element (but keep it out of necessity), make sure ElementSubmit::attach is smart enough to run only once.