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: Creation and deletion of network connection settings.
shell: Move networking onoffbox to top-level.
shell: Use On/Off button for network devices.
shell: Move "Create Connection" to bottom of page.

It's confusing right next to the buttons that activate/deactivate a device.