Improve dropdown button/link support

Problem/Motivation

Core dropdown link/button support should be improved. Here are some examples:

• Admin content type page
  
  
• Node form
  
  

Proposed resolution

Consider using https://v4-alpha.getbootstrap.com/components/dropdowns/ for this, not sure will this work fine when dropdown items are button - this is the case for node form.

Remaining tasks

Do it.