Add setting to disable touchpad when external mouse connected

[davidmhewitt]

Mentioned here: https://www.reddit.com/r/elementaryos/comments/ao5fcz/add_a_feature_on_the_mouse_touchpad_settings/

Happy to implement this if this is a setting that we think should exist.

[rafaelcn]

Hello, I'm the author of that reddit post. My hands are here to help if any help is needed :-)