shell, NM model: Return connection object from add_connection method

Instead of the object path.

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Create a 'ghost' connection settings objects for devices that have none

And make it real as soon as it is changed.

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Move networking onoffbox to top-level

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Use On/Off button for network devices

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Remove other connection settings when enslaving an interface

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Only show most recent connection for a network interface

And ignore the rest.

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Don't mention connection settings in the UI

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Fix tests for previous changes, and remove a race with NM

Closes #977
Reviewed-by: Stef Walter <stefw@redhat.com>