Published on 2020-04-03.

* refactor(web/engine): begins formally removing DOM-aware keyboard API functions from web-core KeyboardInterface (#2915)
* refactor(web/engine): start of system store abstraction (#2919)
* refactor(web/engine): Processor now manages current layer; OSK listens via callback (#2920)
* refactor(web/engine):  RuleBehavior now headless (#2925)
* refactor(web/engine): variable store storage abstraction (#2926)
* fix(web): fixes activeElement typing (#2927)
* refactor(web/engine): relocates DOM-only parts of Processor  (#2938)

Minor fixes and performance improvements