Implement bluetooth tethering support. Use
$KDEDIR/lib/kde4/libexec/networkmanagement_configshell to create a
bluetooth connection like this:

networkmanagement_configshell create -type bluetooth --specific-args
00:11:22:33:44:55,rfcomm0

This command will block until the org.bluez.Device.Connect method for
00:11:22:33:44:55 is executed. After that, it will detect the modem
device from ModemManager (using rfcomm0) and create the bluetooth connection.

Change 00:11:22:33:44:55 to your cell phone's bluetooth hardware address
and rfcomm0 to the string returned by org.bluez.Device.Connect.

Once the connection is created NetworkManager will detect it and enable
the /org/freedesktop/NetworkManager/Devices/<device sequence number> path.

The bluetooth connection and
/org/freedesktop/NetworkManager/Devices/<device sequence number> are not
listed in Plasma NM main window. I will fix that in soon.

The connection is listed in the connection editor, you can change it to
autoconnect or not for instance.