erp5_pdm: use preferred category child item list method in my_variation_category_list field library.

Python script needs to be recompiled when they comes from a change of
class. This was done when business template was installing scripts, but
not when automatic migration from old catalog to new catalog was done.
So make sure to call _compile directly in changeObjectClass to make sure
Python Scripts would always work after a change of class.

Replace the airplane icon by a search button if not value is set on the field.
This make it more usable on mobile, where it's easier to browse than typing text.

Regression introduced by nexedi/erp5@cf1a0610

Calling these can break catalog by not caring about concurrent indexation
activities. Even non-restricted code should not call these, but:
- they cannot be reliably renamed, as there exist activity dependencies
  looking for these exact names (which is a bad practice on its own)
- there is no simple access control mechanism for non-restricted code
So rely on developer discipline instead.

Also, simplify scripts while doing so.

These methods must not be called synchronously:
- they can break catalog by not being careful enough about other
  reindexations which may happen in parallel. See the serialization_tag
  mechanism for more.
- indexation gets executed in the security context of the user causing the
  call, which may lead to an indexation result different from what happens
  when indexation happens with an all-accesses user.

To remove the need to directly call immediate reindexation methods,
allowing to prevent further misuses of these.