Allow disabling keep in background after window closed

When I close the Electrum window, Specter keeps running the background. I need to press Ctrl+C, then it will shut down. I can reliably reproduce this on 1.3.1 on Arch Linux.

There's literally Nothing in the logs when I close the window. Only after Ctrl+C:
```
[2021-05-26 12:28:43,902] INFO in specter: Specter exit cleanup: Stopping Tor daemon
[2021-05-26 12:28:43,903] INFO in specter: Specter exit cleanup: Stopping Tor daemon
[2021-05-26 12:28:43,941] INFO in specter: Closing Specter after cleanup
[2021-05-26 12:28:43,943] WARNING in specter: Failed to connect to Tor control port. Error: [Errno 111] Connection refused
[2021-05-26 12:28:43,945] ERROR in cli_server: Could not initialize tor-system
```

Running processes:

The issue names no source file or test entry point. Start by reproducing the behavior on Arch Linux and tracing what happens when the Electrum window closes; done means providing a way to disable background operation and confirming the process exits without Ctrl+C.

Written by the indexing model from the issue text.