keyfile: merge branch 'th/libnm-keyfile-bgo744699'

Move basic keyfile functionality from settings plugin to libnm-core.
This is a first step to have a semi-standard way to stringify
connections back and forth, which is also available to libnm users.

Still the new functions are internal API (nm-keyfile-internal.h).
Let's decide later how the public API should really look like.

https://bugzilla.gnome.org/show_bug.cgi?id=744699