Simplify HTMLCollection predicates
auto merge of #705 : sonwow/servo/collection, r=jdm

Simplify some APIs that returns HTMLCollection in `dom::Document`.
In order to do this, `dom::htmldocument::createHTMLCollection` is moved to `dom::document::createHTMLCollection`.
Generate bindings for HTMLAppletElement
Generate bindings for HTMLAreaElement
Generate bindings for HTMLBaseElement