* Expose preventDefault and stopPropagation for DOM events
All EventHandler, EventSpec, and EventChain can now carry these extra
"event_actions" that will be applied inside the frontend code when an event is
triggered from the DOM.
Fix#1621
Fix REF-675
* Test cases (and fixes) for "event_actions"
* form: from __future__ import annotations
for py38, py39 compat
* Revert overzealous merge conflict resolution
