A property preferably only emits a notify-changed signal when
the value actually changes and it caches the value (so that
between property-changed signals the value is guaranteed not to change).

NMSettings and NMManager both already cache the hostname, because
NMHostnameManager didn't guarantee this basic concept.

Implement it and rely on it from NMSettings and NMPolicy.
And remove the copy of the property from NMManager.

Move the call for nm_dispatcher_call_hostname() from NMHostnameManager
to NMManager. Note that NMPolicy also has a call to the dispatcher
when set-transient-hostname returns. This should be cleaned up later.