Instead of having NmtEditorPage be a widget itself, have it just be an
object that returns a list of NmtEditorSections, where
NmtEditorSection is a subclass of NmtNewtSection.

(This will be important when adding VPN pages, which will be split up
into multiple sections, but with the different sections needing to
cooperate on updating the NMSettingVpn. This reorganization lets us
have an NMPageVpn containing multiple sections, with the NMPageVpn
object handling the coordination between the sections.)