Bugfix/improve form widgets

In this MR, 3 commits, each one corresponding to one change:

1. Improve syntax for range widgets:
   • Invert the order of parameters for widgets using range (widgets/form.js).
   • Change the parameter of the sib-widget from data-parent on the parent template, to data-range on the child template.
2. Allow to use a callback after the render of a widget. With this callback, we can initialize the choices library for the autocomplete field. NB: should we move this widget in another file to import the library only when needed?
3. For the placeholder dropdown widget, we force the selection of the placeholder when value == "", which means no value is defined for the field