Connection editor does not open if gnome is installed, but user is not using gnome

[lugeha]

https://github.com/firecat53/networkmanager-dmenu/blob/0bd2f11fdc6c028bada5c54e67051124631278fe/networkmanager_dmenu#L749

If you have gnome installed and the user is not in a gnome session, the connection editor will not launch as it tries to use gnome-control-center.

#136 allows you to select which gui editor to use.