Add new `css_generic_link_active_color` field to use on active tab (tabbed changeform).

[fabiocaccamo]

Yes, adding a css_generic_link_active_color would solve any doubt and would also be available for possible other future cases.

Originally posted by @fabiocaccamo in #231 (comment)

[fabiocaccamo]

@mjwalter field available in 0.25.0 version.