SVGHMI: JS refactoring continued : "dispatch_value_to_widget" becomes widget class member "new_hmi_value" (was also broken by previous relativeness commit)

SVGHMI: widgets are not anymore binary relative or absolute, but have a "relativeness".

Because of allowing multiple variables per widget, we must distinguish if individual variables is relative to page, not the whole widget.

SVGHMI: More JS code refactoring : change_hmi_value and apply_hmi_value now methods of widget class.

SVGHMI: Better JS code semantic in ForEach widget class. Update lagging generated xslt.

SVGHMI: Add ForEach Widget is svghmi test

SVGHMI: add get_idx(index) to widget objects to factorize a bit code repeated accross widgets. To be continued.

SVGHMI: prefer apply_hmi_value() to change_hmi_value() when possible

SVGHMI: Add RelativePages to tests

SVGHMI: Update Widget Display templates to generate Class based declaration.

SVGHMI: JsonTable now picks items from HMI:List, and update texts, all according to Json data. Still miss scrolling.

SVGHMI: JsonTable now generate working data access code for data/* elements.

SVGHMI: Minimal support for HMI:List : clones pointing to elements of any HMI:List are never unlinked, widgets using HMI:List will rely on this.