NMConnection is an interface, which is implemented by the types
NMSimpleConnection (libnm-core), NMSettingsConnection (src) and
NMRemoteConnection (libnm).

NMSettingsConnection does a lot of things already:

  1) it "is-a" NMDBusObject and exports the API of a connection profile
     on D-Bus
  2) it interacts with NMSettings and contains functionality
     for tracking the profiles.
  3) it is the base-class of types like NMSKeyfileConnection and
     NMIfcfgConnection. These handle how the profile is persisted
     on disk.
  4) it implements NMConnection interface, to itself track the
     settings of the profile.

3) and 4) would be better implemented via delegation than inheritance.

Address 4) and don't let NMSettingsConnection implemente the NMConnection
interface. Instead, a settings-connection references now a NMSimpleConnection
instance, to which it delegates for keeping the actual profiles.

Advantages:

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