Implement Shadow DOM support

[spmonahan]

Tabster relies on several DOM APIs like TreeWalker, Node.contains() and Node.parentElement that do not look into shadow roots. This prevents Tabster from working with custom elements that use the shadow DOM.

This issue is a parent for several issues to bring shadow DOM support to Tabster. Issues are roughly in priority with the highest being at the top.

• Shadow DOM: Update MutationObserver to work with shadow roots #335
• Shadow DOM: implement TreeWalker that can walk shadow roots #338
• Shadow DOM: implement Node.contains() function to support shadow roots and slots #337
• Shadow DOM: implement a "getParent()" function to correctly walk up the DOM tree for elements in the shadow root #339
• Shadow DOM: support activeElement in shadow roots #336

[spmonahan]

Related: #341