Remove not-yet-implemented members from IDL and Rust code.

These stub implementations bring no value, and significantly increased
maintenance costs.

This commit also synchronizes the IDL with the HTML specification and
makes the URLs consistent.
Restore HTMLSelectElement.add to avoid neutering test_union.html.
auto merge of #2543 : Ms2ger/servo/nyi, r=jdm

These stub implementations bring no value, and significantly increased
maintenance costs.

This commit also synchronizes the IDL with the HTML specification and
makes the URLs consistent.