bwm-ng: move bwm-ng from "Networking applications" to "System tools".

bwm-ng can monitor not only network bandwidth but also disk-io
bandwidth, so it is more appropriate to put bwm-ng into System tools.

Signed-off-by: Kelvin Cheung <keguang.zhang@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>