Summary:
This moves Formatter and Unit out of ProcessCore and into its own Formatter library.
This allows us to reuse them in other places, like a new sensor library that will be
posted later.

It includes a declarative plugin that makes things available to QML.

It also contains a small unit test for Formatter.

Test Plan:
- The new unit test passes.
- KSysGuard still runs correctly.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: zzag, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27869