settings: implement ability to add connections without saving them to disk
We don't always want to immediately write new connections to disk, to facilitate "runtime" or "temporary" connections where an interface's runtime config isn't backed by on-disk config. Also, just because an interface's configuration is changed doesn't necessarily mean that new configuration should be written to disk either. Add D-Bus methods for adding new connections and for updating existing connections that don't immediately save the connection to disk. Also add infrastructure to indicate to plugins that the new connection shouldn't be immediately saved if the connection was added with the new method.
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