shell, NM model: Return connection object from add_connection method.

Instead of the object path.
shell: Create a 'ghost' connection settings objects for devices that have none.

And make it real as soon as it is changed.
shell: Move networking onoffbox to top-level.
shell: Use On/Off button for network devices.
shell: Remove header of network connection setting panel.

It is pointless when there is only one connection setting.
shell: Remove other connection settings when enslaving an interface.
shell: Don't mention connection id in links.
shell: Only show most recent connection for a network interface.

And ignore the rest.
shell: Fix tests for previous changes, and remove a race with NM.