Widget dropdown - select

With the new CheckboxWidget in, work on this should be easier.

Working on this.

Here is a working dropdown.

Some notes:
1. I used URLs as the select option values. That will make possible to use JS to go to URL directly rather than submit form. (I saw this in D7 and found it pretty useful)
2. Dropdown only works well with the show_only_one_result mode enabled. Probably we also need form validation for this.
3. I'm not sure if I will have time to work on this issue or on #2625188: Allow limiting to one active item.

This looks great, I'll write a unit test and integration test for this as well so we can get this in.

I have done some work on this, but I don't have enough time now to finish this. I'll come back to this issue later today or tomorrow. Anyway - saving work.

Tests should be green now.

Interesting, checkbox widget uses form build info to inject and retrieve the facet, but here it works from a constructor. Probably this can be removed?

I think this way is a little bit cleaner and we should create a followup to make the checkbox look like this one, would you agree?

Found a small bug: user may be redirected to /_none path.

Created a followup for #10: #2681053: Checkboxes widget: facet injected to form in two different ways

The code here looks great and I think we can commit this and there's a bunch of test coverage added for this. If there are problems with this patch we'll get issues filed for them so we can add specific test coverage for that as well.

Thank you very much for your work on this!