web, NM Model: Use "priv(obj).foo" notation instead of obj[' foo'].

This is less obscure, more robust, and indeed a bug was found.

(Also, 'type' is no longer optional with get_object).
agent: Allow fake object manager to be idle.

This is useful when it isn't needed at all.
web, NM Model: Don't use the fake object manager.

This is to show what is needed to make the model work.

The fake object manager can act as an optimization to this (by
providing properties before we can ask for them and thus avoiding
round-trips), but it might be better to make that more explicit by
having the agent send incremental "seed" messages.
web, NM model: Don't ask udev for non-udev devices.
web, NM model: Restructure code to better show the individual parts.