It is a hook to set one or several D-Bus properties at once. Properties that are
to be set-able, have to be whitelisted in the stub.

Previously, the callbacks would return gl.mainloop.quit(), which is None
and thus the source got removed already. The later GLib.source_remove()
would thus use an invalid source-id.

Fix that, by no removing the source id from the callback.

Fixes: b1ff82eba5e80e4ef3f16e35372ba933c300be1d

Instead of exporting exported-objects during construction, export
them right before we register them (for example, in case of device
instances, before we add the device to the manager's device list).

Likewise, when removing the object from the list, always explicitly
unexport them.

Without explicitly unexporting the objects, they can also not be
removed, because object-manage and D-Bus library keep references
to them.

Instead of registering a __get_props() function which constructs the
entire hash anew each time, register a props dictionary. This dictionary
is used as cache for the properies.

In most cases, we don't need to additionally cache the properties
outside the props dictionary. Also, add _dbus_property_set() which
sets the property and emits the notification.

Also, cleanup the property names by giving them a PRP_ prefix.

Transition the states, so that `nmcli con up` can succeed.

- no more global variables, except those in the new variable "gl".

- don't pass that bus instance around. Use the singleton gl.bus.

- separate creation of ExportedObj from exporting on D-Bus.

- use enum values loaded from NM via GObject introspection.

- the visible change is that the generated D-Bus paths now start
  counting at one. That is also how NetworkManager behaves, and
  it looks nicer to have no zero ID for an object.

- add find_devices() and find_device_first() functions,
  to not re-implement iterating the device list.

- for test functions, accept the device's "ident", instead
  of ifname. The "ident" must b unique, contrary to the "ifname".