Remove unused InitIds function.
auto merge of #2637 : Ms2ger/servo/initids, r=larsbergstrom
auto merge of #2633 : Ms2ger/servo/derefmut, r=jdm

Part of #1854.
Use interior mutability for MouseEvent.
Use internal mutability for Document::idmap.
Pass immutable self to Node::{SetNodeValue, SetTextContent}.
Pass immutable document to parse_html.
Cleanup a mutable variable in dispatch_event.