Align activation behavior with changes to DOM Standard

There are two major changes here:

* Any click event can cause activation behavior to run. No exceptions.
* The "click in progress flag" is now restricted to the click() method, as it already is in Firefox.

The other changes are editorial:

* "Activation behavior" is now associated with an object at all times (and cannot be conditionally associated)
* "Legacy-pre-activation behavior" and "legacy-canceled-activation behavior" are now more clearly only relevant for input elements in either the Checkbox or Radio Button state.
* Various concepts such as "run synthetic click activation steps" and "nearest activatable element" are gone as they are obsoleted by the DOM Standard or the aforementioned normative changes.

Tests related to this change:

* web-platform-tests/wpt#3974
* web-platform-tests/wpt#4034

Changes to the DOM Standard that were necessary for this change:

* whatwg/dom#342
* whatwg/dom#346

Fixes whatwg#1394.