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.

Do not check portal's isIndexable, it is checked again in ZSQLCatalog.
Directly bind superclass callable instead of doing an extra getattr +
call.
Trivial python simplifications.
Assorted coding style improvements.
Drop unhelpful comments and docstrings.