feat(HXElement): make all emitted events cancelable

* Allows any emitted event to be cancelable.
* Return status from `$emit()`
  * If `true`, the event was not canceled.
  * If `false`, the event was canceled by an event listener.

Author: Ryan A. Johnson

src/helix-ui/elements/HXElement.js

@@ -88,10 +88,11 @@ export class HXElement extends HTMLElement {
 
     $emit (evtName, details) {
         let evt = new CustomEvent(evtName, {
+            cancelable: true,
             bubbles: true,
             detail: details,
         });
-        this.dispatchEvent(evt);
+        return this.dispatchEvent(evt);
     }//$emit
 
     $relayEvent (oldEvent) {