add ability to select gui

[lugeha]

If gnome is installed, but you are not in a gnome environment, gnome-control-center is still used. However, it will not work outside of gnome shells.

This PR adds the ability to select which GUI editor to use, the fallback value is still gnome-control-center.

However, this will break for people who do not have gnome installed as it will not fall through to nm-connection-editor.

Configuration example is as follows,

[editor]
gui = nm-connection-editor