Split breakpoints into API and UI module

This module needs to be split into two parts:

1. Breakpoints will become an API module containing all the functionality to work with breakpoints
2. Breakpoints UI will be a new (sub) module to provide a UI to site administrators

Related issues:

• #1734642: Move breakpoint module into core
• #1775530: Move picture into core
• #1775774: Allow themes to identify their breakpoints to Drupal