add super.dispose, add oDispose to API

src/common/Types.d.ts

@@ -183,6 +183,7 @@ export interface IMarker extends IDisposable {
   readonly id: number;
   readonly isDisposed: boolean;
   readonly line: number;
+  onDispose: IEvent<void>;
 }
 export interface IModes {
   insertMode: boolean;

src/common/buffer/Marker.ts

@@ -32,5 +32,6 @@ export class Marker extends Disposable implements IMarker {
     this.line = -1;
     // Emit before super.dispose such that dispose listeners get a change to react
     this._onDispose.fire();
+    super.dispose();
   }
 }