ChoiceFilterState improve init

[nikolas-burkoff]

I think these get_choices_label functions are too complicated. I think we can simplify and tidy up the way how we obtain counts. In the init of choicesFilterState there is private$histogram_data which isn't used at all. Having private$choices where value is a factor level and name is a count is also not elegant. I think we can introduce

private$choices # factor levels
private$choices_count # unfiltered count
private$f_choices_count # reactive
private$calculate_choices_count(x) 

That is just a proposition, I think it would be nice if we tidy up these initialize methods and how we preserve counts.

Originally posted by @gogonzo in #180 (comment)

[donyunardi]

Acceptance Criteria:

• Simplify the property/attributes of the class
• Update the class' methods