DocumentFragment.getElementById() not implemented

[balmoral]

DocumentFragment.getElementById() is not implemented for Browser::DOM::DocumentFragment

https://developer.mozilla.org/en-US/docs/Web/API/DocumentFragment

[hmdne]

DocumentFragment implements ParentNode which doesn't implement getElementById (this may change in the future though). We wrongly assume at this point, that DocumentFragment inherits from Element, but actually it inherits from Node. A robust implementation should implement a module ParentNode which would be included by both Element, DocumentFragment and Document (and make them all inherit from Node).

See #46