Root slider element: add custom class

[becquerel]

ability to add custom class to slider, so it can be styled based on class, not just ID

[arturmamedov]

We can create a text field in db, with a json content called extra_json, cast it in Entity to be formatted to array and add all settings that we wont, defined in a config file, to slider with json { attr=value } format.

Similary to the Settings module, infact we need to use some staff from it, how a view of the fields of different types (checkbox, text, textarea ...).