hostname: cache hostname-manager's hostname property
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.
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