connection-editor: Expose metered connection switch

NetworkManager supports marking a connection as metered, meaning
that connection is e.g. charged by its usage or extremely limited.
When a network is metered, background network usage must cease
and the behavior of the various OS pieces must be adjusted.

This patch adds a switch to set a connection as metered or not.
The property is imediately propagated to NetworkManager.

https://bugzilla.gnome.org/show_bug.cgi?id=792608