network: Handle cloned-mac setting
This can be set to 'preserve', 'permanent', 'random' or 'stable'. We need to handle these values otherwise we can end up with the editor being un-saveable.
Turn the entry in to a GtkComboBoxText with those items in it, allowing a mac address to be typed too.
Partial copy of 85b6b659a140a59c3df787062e089a0b4e2a547d from network-manager-applet.