Add Total/Available/Allocated memory sensors

Summary:
This exposes the previously hidden Total memory amount as
"mem/physical/total".

It also add an "Available" sensor, which reads the "MemAvailable" field
from /proc/meminfo. "Available" provides an estimate with how much
memory is available for starting new applications.

Finally, it adds an "Allocated" sensor, which equal "Total - Available"
and represents the total amount allocated to all currently running
applications.

Test Plan: The sensor browser now lists "total", "available" and "allocated" sensors.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25305