Draft: main/postmarketos-base-ui-networkmanager: use a second usb-tethering connection

This is a follow-up of IRC discuss.

I'm investigating on USB Tethering mode for Sxmo, and how to help our users to enable this mode.

Pmos rely on a NetworkManager dispatcher script to deal with tethering. Atm it manage its owned system network manager connection. It check it ipv4.method value, and toggle the tethering.

This is a proposition to use multiple system connections, so that a user could switch from one to another mode without editing the connections themselves.

My only question atm is that if a user does edit those connections, the dispatcher will not fix them anymore for them. Problem or not, debatable.

related to: !3819 (merged)