clean up the EntityOperationsOperationAddGeneric form hack

EntityOperationsOperationAddGeneric contains a bit of a hack, as it must show either:

- a list of links for the entity types to create
- the actual form

It does this by generally claiming to be a form, and for the first case just returning a list of links.

This is fine as far as the UI goes, but for form alteration it's going to cause a WTF.

The fix is most likely to change the page callback it registered in its menu_item() to go in via the page callback and get to build(). The build() method can then either return page content, or return the result of a drupal_get_form() -- which would go back into the handler and get its form().