Add per-partition total and all partitions sensors

Summary:
This adds a "total" sensor for each partition that reports the total
space for that partition. It also adds a new "all" category with sensors
that report the used/free space for all partitions.

Test Plan: New sesnors are available in KSysGuard

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28438