Split up the InputButton variant
Implement form control mutability, rename FormOwner -> FormControl
Add stub Activatable trait
Add trusted setter to Event
Implement basic (unhooked) framework for element activation
Hook up authentic click activation to the script task
Some reorganization of activation code:

 - Make method name apply to trait implementor better (When a user agent is to run post-click activation steps on an element, it must run the activation behavior defined for that element)
 - Mention invariants and conditions on authentic_click_activation
Move InputSubmit to Activatable
Move InputCheckbox to Activatable
Refactor code for fetching radio siblings
Move InputRadio to Activatable
Hook up synthetic click activation to script_task and <>.click()
Implement implicit form submission
Address review comments
Implement QuerySelectorIterator (fixes #4082)
Switch over to query_selector_iter
Switch to NodeFlags (the footprint has not changed)
Add oninput/onchange so tests work
Add Comparable trait to js.rs; fixups
auto merge of #4002 : Manishearth/servo/activation, r=jdm

Still need to impl `Activatable` on all activatable elements. I'll probably push those changes to this PR, however they can be made separately as well.