shell: Migrate networking code to dbus-json3

This is the first step to moving the Networking page into a component.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Remove support for dbus-json2

It's no longer in use now that the networking code has been
migrated to dbus-json3

We also no longer need the CockpitFakeManager code since all of
the dbus-json1 callers are interacting directly with ObjectManager
based services.

Fixes #1388
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
shell: Don't rely on remove notifications for dissappearing NM objects

The dbus-json3 notify code is a lot less intense about aggressively
introspecting every object after every method call, which was a big
performance issue. So we have to know when stuff goes away and clean
it up ourselves.

Closes #1511
Signed-off-by: Stef Walter <stefw@redhat.com>
 * Updated commit message