Fix Ctrl+D Crashes and Freezes (#296)

Cleaned version of https://github.com/p-e-w/finalterm/pull/306
Of course, this timeout leaves the issue I described: Close a widget with the "X", then quickly press `Ctrl+D` on another one – the `SIGCHLD` from both will fall within the `close_in_progress` window, and the second widget will be left open :disappointed_relieved:. I don't think it will be possible to eliminate such issues while keeping timeouts anywhere.