Move helper classes from basic.css to a single css file

A list of helpers classes has been added and completed in the css rules list in the platform since the 5.5 version.
The goal of these classes is to modify or add one css rule to an element, in a specific case.

Example: the .foldableBox style is a box with a grey header and a border.
If I need this box to have a centered content, I want to keep these styles + add my helper textAlignC rule.

The helpers classes need to be loaded at the end of the css list, just before the custom css file, to be sure to not be overriden.

Task to do: split basics.css to 2 files: basics and helpers, and declare the helpers.css in the platform & addons.