[dataimport] methods that modify in-place shouldn't return value
Also mark those methods private and drop some useless comments in passing.
Related to #5414760
CSS Coding Standards
--------------------
(Draft, to be continued)
:Naming: camelCase
Indentation rules
~~~~~~~~~~~~~~~~~
- 2 espaces avant les propriétés
- pas d'espace avant les ":", un espace après
- 1 seul espace entre les différentes valeurs pour une même propriété
Documentation
~~~~~~~~~~~~~
Please keep rules semantically linked grouped together, with a comment about
what they are for.
Recommendation
~~~~~~~~~~~~~~
- Try to use existing classes rather than introduce new ones
- Keep things as simple as possible while in the framework
- Think about later customization by application
- Avoid introducing a new CSS file for a few lines of CSS, at least while the
framework doesn't include packing functionalities