Use same format for all events and emitters #4164
Description
Currently there are a couple of patterns used for events:
private _onBinary = new EventEmitter<string>();
public get onBinary(): IEvent<string> { return this._onBinary.event; }and
private _onDispose = this.register(new EventEmitter<void>());
public readonly onDispose = this._onDispose.event;We should standardize on the second as it's simpler and avoids the getter at runtime.