Code style
Revamp UI for smaller code base and move features

* Switch from Stylus to SASS due to the need to extended inheritance for
  selectors. When using a selector `.a` it will be expanded also with
  all selectors extending `.a`.
* Add button group.
* Revamp Rdb.key utilities usign simple fn chain instead of complex
  scopes.
* Rewrite dropdown JS code with position and positioning depending on
  available viewport space (left/right above/below button)
* Add icon font.
* Add separate theme stylesheet containing most colors etc.