Add a per-process network information plugin

Summary:
This adds a ksysguard process plugin that provides information about
how much data is being used by a certain process. Since there is no
unpriviledged API available (yet?) for this information, a small
helper application is used that uses libpcap to capture packet sizes,
which is then mapped to individual processes by comparing sockets to
entries in /proc.

The helper application is intentionally kept small and should not
require anything else than `cap_net_raw`.

Depends on D23287

Test Plan:
Ensure ksgrd_network_helper has cap_net_raw set, by running
`setcap cap_net_raw+ep ksgrd_network_helper`. After that, start
ksysguard. The "upload" and "download" columns should be
automatically enabled and display network traffic for a certain
process.

Reviewers: #plasma, meven

Reviewed By: meven

Subscribers: meven, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23342