Until now, only bt5 Extension/Test/Document could be migrated from
filesystem.

From migration dialog, allow to select any Products.ERP5.Documents.* (only,
for now) to be migrated. By default, automatically select Products Documents
used by the current bt5 Portal Types (by looking at the mro() of its
erp5.portal_type classes).

XXX-BEFORE-MERGE: Add Unit Test.