Prevent overriding of Disposable.dispose

A common mistake made was overriding dispose and not calling super.dispose,
so a TS trick is now used to move it to a readonly property instead of a
member method which prevents it being overridden.

src/browser/ScreenDprMonitor.ts

@@ -3,7 +3,7 @@
  * @license MIT
  */
 
-import { Disposable } from 'common/Lifecycle';
+import { Disposable, toDisposable } from 'common/Lifecycle';
 
 export type ScreenDprListener = (newDevicePixelRatio?: number, oldDevicePixelRatio?: number) => void;
 
@@ -26,6 +26,9 @@ export class ScreenDprMonitor extends Disposable {
   constructor(private _parentWindow: Window) {
     super();
     this._currentDevicePixelRatio = this._parentWindow.devicePixelRatio;
+    this.register(toDisposable(() => {
+      this.clearListener();
+    }));
   }
 
   public setListener(listener: ScreenDprListener): void {
@@ -43,11 +46,6 @@ export class ScreenDprMonitor extends Disposable {
     this._updateDpr();
   }
 
-  public dispose(): void {
-    super.dispose();
-    this.clearListener();
-  }
-
   private _updateDpr(): void {
     if (!this._outerListener) {
       return;