fix #1085 checkbox init in MAC dropdown

checkboxes related to selected options are not ticked when the widget is initialised.
fix #1085 checkbox init in MAC dropdown

checkboxes related to selected options are not ticked when the widget is initialised.
fix #1082 'Tab' doesn't behave as 'enter' when defined as a selection key
feat #1086 Deprecation of cssClass in sections and repeaters

`cssClass` property in section and repeater configuration is now deprecated. It has to be replaced by `attributes.classList`.

In the specific case of the `childSections` configuration for the repeater, the `cssClass` could be a function like any other property, with the peculiarity that the function was called every time a change occurred in the bound map/array. This allowed to implement pyjama tables.
With this commit, this feature is replaced and enhanced: if the `attributes` property in the `childSections` configuration is a function, it is called every time there is a data change and its returned value is used to update the section attributes of each entry.

Close #1086