Add a simple network bandwidth monitor (uses libgtop as a fallback)

The implementation uses /proc/net/netstat by default (if it is available) because it is more efficient than using libgtop. It takes about 250 syscalls for libgtop to obtain information about a couple of network interfaces, while it takes just a few syscalls to read /proc/net/netstat.

A full network bar (100% network load) corresponds to a constant of 100 Mbit/s. Bandwidths higher than 100 Mbit/s are clamped to 100%. The tooltip however is showing non-clamped bandwidth rounded to Mbit/s.

Closes: #14 (closed)