Completely rework how settings plugin handle connections and how
NMSettings tracks the list of connections.

Previously, settings plugins would return objects of (a subtype of) type
NMSettingsConnection. The NMSettingsConnection was tightly coupled with
the settings plugin. That has a lot of downsides.

Change that. When changing this basic relation how settings connections
are tracked, everything falls appart. That's why this is a huge change.
Also, since I have to largely rewrite the settings plugins, I also
added support for multiple keyfile directories, handle in-memory
connections only by keyfile plugin and (partly) use copy-on-write NMConnection
instances. I don't want to spend effort rewriting large parts while
preserving the old way, that anyway should change. E.g. while rewriting ifcfg-rh,
I don't want to let it handle in-memory connections because that's not right
long-term.

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If the settings plugins themself create subtypes ...