Added CollectionFilter, CollectionTypeId
Implemented HTMLCollection::create_live
Support live HTMLCollection for Document collections

The following interface methods now return live collections:
Document.{images,embeds,plugins,links,forms,scripts,anchors,applets}
Support live HTMLCollection for FieldSet.elements
Support live HTMLCollection for DataListElement.options
Updated HTMLCollection content test
HTMLCollection::create returns live collections by default
auto merge of #1952 : brunoabinader/servo/htmlcollection-live, r=Ms2ger

Closes #1662.

PS: Cached collections will be next step.