Nowadays, users should use the standard "PropertiesChanged" signal
on "org.freedesktop.DBus.Properties" interface.
(cherry picked from commit 6fb917178a)
Since libnm is the preferred way to interact with NM now, we don't
want to add new device types to libnm-glib.
Make libnm-glib recognize TUN devices as generic ones and modify
NMDeviceGeneric to use the correct D-Bus interface based on the actual
device type.