Add a new libnm-glib method to get the type description for a device,
and use it in nmcli. For most types, the type description is based on
the class name, but for NMDeviceGeneric, it comes from the
:type-description property.
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.
