[w3c/webpayments-payment-handler] Merge PaymentAppResponse and PaymentRequestEvent (#142)

I see no reason to have both of these. 
The PaymentRequestEvent should be updated to look like this:

```
interface PaymentRequestEvent : ExtendableEvent {
    readonly attribute DOMString topLevelOrigin;
    readonly attribute DOMString paymentRequestOrigin;
    readonly attribute DOMString paymentRequestId;
    readonly attribute FrozenArray<PaymentMethodData> methodData;
    readonly attribute PaymentItem total;
    readonly attribute FrozenArray<PaymentDetailsModifier> modifiers;
    readonly attribute DOMString instrumentId;
    Promise<WindowClient> openWindow(USVString url);
    void respondWith(Promise<PaymentHandlerResponse> response);
};
```
And `PaymentAppResponse` should be renamed to `PaymentHandlerResponse`.

This would also address @mattsaxon 's comments on #127 

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