Enqueue transfer requests (to prevent accepting them all without review)

[nekohayo]

Here's a small security issue. If you have three computers, and from two of the three you throw files at the third one at roughly the same time, the third computer will be prompted only once to accept the (first) transfer, but the 2nd transfer will get accepted along with them without review.

[sireliah]

I couldn't reproduce this locally on one machine now, but I suspect the issue is here.

That is:

1. Sender ---- ([Metadata] I'm sending you hash xxxx) ----> Receiver
2. Receiver (gets the user approval for the hash xxxx)
3. Sender <--- ([Metadata] I'm accepting hash xxxx) ----- Receiver
4. Sender ---- (sends whatever on the opened substream) ----> Receiver
5. FIN

• On the point 4. we might check what the Receiver actually accepted and send only the matching payload over the substream.

Although this doesn't solve the underlying problem with the receiver acceptance flow. I'll try to dig this a bit more.

Meanwhile while testing 3 peers I discovered that sending payloads from two peers to third one, the notification popup is not closed and prevents you from using the application, so you need to kill the app.

• Fix the bug with Yes/No popup not being closed.