fix: fix redundant event dispatch (#599)

Including  multiple instances of the webcomponents on the same page
causes the global event handlers to be registered multiple times
This then causes multiple calls of the event handlers on the prototype
and multiple semantic events being fired.

Author: pskelin

packages/base/src/DOMEventHandler.js

@@ -49,14 +49,25 @@ const getParentDOMNode = function getParentDOMNode(node) {
 	return parentNode;
 };
 
+const isOtherInstanceRegistered = () => {
+	return window["@ui5/webcomponents-base/DOMEventHandler"];
+};
+
+const registerInstance = () => {
+	window["@ui5/webcomponents-base/DOMEventHandler"] = true;
+};
 
 class DOMEventHandler {
 	constructor() {
 		throw new Error("Static class");
 	}
 
 	static start() {
-		ManagedEvents.bindAllEvents(handleEvent);
+		// register the handlers just once in case other bundles include and call this method multiple times
+		if (!isOtherInstanceRegistered()) {
+			ManagedEvents.bindAllEvents(handleEvent);
+			registerInstance();
+		}
 	}
 
 	static stop() {