libnm: port NMRemoteSettings to NMObject

libnm: port NMRemoteSettings to NMObject

NMRemoteSettings duplicates a bunch of NMObject's functionality that
it doesn't need to. In libnm-glib, it wouldn't have been possible to
port NMRemoteSettings to NMObject without breaking ABI, but now in
libnm we can do that.

As a side effect of this, NMRemoteSettings gains a "connections"
property, and "connection-added" and "connection-removed" signals
(with the former replacing the old "new-connection" signal). This also
removes the "connections-loaded" signal, since the connections will
now always be loaded (via the initialization of the "connections"
property) during init()/init_async().

Also, this removes NMRemoteConnection's "removed" signal, since it's
redundant with the new NMRemoteSettings::connection-removed (and
having the signal on NMRemoteSettings instead is more consistent with
other objects).