Nowadays, users should use the standard "PropertiesChanged" signal
on "org.freedesktop.DBus.Properties" interface.
(cherry picked from commit 6fb917178a)
Add NMDeviceGeneric, to provide generic support for unknown device
types, and create NMDeviceGenerics for those devices that NM
previously was ignoring. Allow NMSettingGeneric connections to be
activated on (managed) NMDeviceGenerics.