Add an option to hide kernel threads

[yahlia]

Checklist

• I've looked through the documentation and existing open issues for similar feature requests.

Describe the feature request

Add an option to hide kernel threads like you can in htop.

• SHIFT+K toggles between show/hide kernel threads in the process widget.
• Default should be to hide them, as you're usually not interested in kernel threads.
• Command line options should look like -K, --hide-kernel.