Qt interface delete sent handling

[PeterSurda]

If a message is in queued or doing PoW, it shouldn't be possible to delete it. It can create problems.

Alternatively, the singleWorker thread can check for the message still being in the "sent" folder during intermediary steps.

[g1itch]

I deleted a message from "Sent" while BM was doing PoW and nothing happened.

[navjotcis]

I think this issue is a duplicate of #1505 issue