fix: Refactor DOMElement

* No longer keep track of unnecessary state (e.g. uiEvents)
* Fix onMount
* Allow adding the DOMElement via Node#addComponent instead of DOMElement
  constructor
* Improve (basically rewrite) test suite
* Allow recycling of DOMElement
* Store state in external Spec (consistent with Node)
* Make DOMElement#getValue consistent with spec naming conventions
  (e.g. styles/ id)
* Add missing getter functions
* Fix documentation
* Obsolete/ Remove DOMElement#draw DOMElement#init (users called it
  explicitly due to confusion)
* Fix return values described in the docs (make methods chainable)
* Properly reset ElementCache when elements are being reused
* Generalize dismounting: preserve DOMElement spec
* Reset properties, attributes etc. onDismount