Add KSysGuard::Sensors library

Summary:
This adds a new library for communicating with a new KSysGuard
DBus-based daemon that provides sensors. The daemon will be
posted by d_ed later on.

This new library pretty much maps the different objects from
the daemon to C++ objects.

Depends on D28333

Test Plan: None yet, needs the new daemon.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: ngraham, mart, zzag, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28141