Add simulated Market purchase events in the Editor

When testing in the editor, the "Buy" events fired when PurchaseType is PurchaseWithVirtualItem get run. However, when PurchaseType is PurchaseWithMarket, no "Buy" events are run or simulated. This blocks testing event handling while in the editor.

I've added simulated onMarketPurchaseStarted and onMarketPurchase events for when run in the editor. In the editor we'll assume that all market purchases succeed.

While it would be nice to have some way of simulating cancelled market purchases, I don't have a clean way of simulating that event.