GDBusProxy: Remove error in get_cached_property() and add set_cached_property()

GDBusProxy: Remove error in get_cached_property() and add set_cached_property()

This makes it possible to use the cached properties mechanism even if
constructing the proxy with the DO_NOT_LOAD_PROPERTIES flag.

This is useful for cases where you obtain the and track object
properties out-of-band. For example, in udisks, the plan is to have
something like this

 Manager.GetObjects    (out ao paths, out aa{sa{sv}} all_properties);
 Manager.ObjectAdded   (o path, a{sa{sv}} all_properties);
 Manager.ObjectChanged (o path, a{sa{sv}} all_properties);
 Manager.ObjectRemoved (o path, a{sa{sv}} all_properties);

E.g. the first GetObjects() call will return *all* data about *all*
exported objects. Further, this way a client will only need to listen
these three signals (three AddMatch) on the Manager object and it will
never need to do GetAll() etc (e.g. can use DO_NOT_LOAD_PROPERTIES).

(Of course this only works if the client is interested in all
objects... while this is true for udisks it is generally not true for
other D-Bus services).

Also use expected_interface to check for programming errors.