Fix stop delay and threading error

[Chapoly1305]

For Issue #58
Tested on Macbook and Ubuntu 22.04, the script is now responsive when press stop button without giving a pop-up error.

The previous pull request (#64) was not merged but the head brach was deleted by accident. Creating a new pull request with identical modification.

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[e-rk]

I have gave the change a try and it looks like there is a problem with stopping the capture on Linux. When the Stop capturing packets button is clicked, the extcap does not close the serial port and the process does not terminate.
Unfortunately the code around the cleanup is very flaky and difficult to get right on every operating system.

[Chapoly1305]

@e-rk Hi, thanks for the test. From your description, it sounds you have tested on the wrong file? Your description matches exactly the old script, and that's why I modified the previous script. I have personally tested on Linux and other users reported positive feedback on #58, So I kindly ask your favor to try again, ensure you replaced the old file and are using the modified script. If you may, remove the existing files from local and global, then add this new python file. Thanks.