Move Dom listener helper to ui/

Part of #1518

src/utils/Dom.ts

@@ -1,31 +0,0 @@
-/**
- * Copyright (c) 2018 The xterm.js authors. All rights reserved.
- * @license MIT
- */
-
-import { IDisposable } from 'xterm';
-
-/**
- * Adds a disposabe listener to a node in the DOM, returning the disposable.
- * @param type The event type.
- * @param handler The handler for the listener.
- */
-export function addDisposableListener(
-  node: Element | Window | Document,
-  type: string,
-  handler: (e: any) => void,
-  useCapture?: boolean
-): IDisposable {
-  node.addEventListener(type, handler, useCapture);
-  return {
-    dispose: () => {
-      if (!handler) {
-        // Already disposed
-        return;
-      }
-      node.removeEventListener(type, handler, useCapture);
-      node = null;
-      handler = null;
-    }
-  };
-}