dbus-json: Allow working with objects without proxy.

The client might not want proxies in the agent and in DBusClient for
all objects.  This commit adds the 'signal' event that is emitted for
any signal from the service, and the 'call' method for calling methods
on any object path and interface name.
web, NM model: Don't use DBusClient proxies.
web, NM model: Optionally use a fake object manager as an optimization.

To save round trips and get rid of obsolete objects.