create game UI modification to allow button limiting by button set, TL status, or die skills

* Fixes #109 (we can open new tickets for other requested changes)
* Jenkins: http://jenkins.buttonweavers.com:8080/job/buttonmen-cgolubi1/254/
Okay, this encoding of skills is inconsistent with the encoding of die skills, i.e., the name of the skills as keys, and the skill class names as values.

I also note that you have already used
  $dieSkills = array_keys($button->dieSkills)
in BMInterface line 1933

so using a consistent encoding of lists of skills would not break your code further on.

I would recommend something like:
  $this->dieSkills += $die->skillList;
instead of the foreach here.