Implement sorting of the device tree items

This patch fixes the issue of processor numbers not being sorted in human-readable order.

It overloads the < operator on the SolDevice class to implement sorting by process number and sorting by storage volume names (as they
should go, e.g.: sda, sda1, sda2...).

BUG: 380355

Reviewed by: cfeck, davidedmundson

Differential Revision: https://phabricator.kde.org/D7155