Fix reports of memory usage on Linux and macOS

Summary:
The Workbench was far off reporting the process memory in the status bar for both Linux and macOS.

I changed the calculations for both cases according to well-tested and proven software like the
GNOME system monitor and Chromium.

I also did some maintenance and modernization work on involved classes, like removing unused
function templates and methods, converting a solely static class into a namespace and so on...

Fixes T29342

Test Plan:
I tested the reported memory on Windows 10, Ubuntu 20.04, 22.04, as well as on macOS Catalina, Big Sur, and Monteray.
The reported amount of memory did match the memory reports of GNOME system monitor and Chromium.

Reviewers: O3 MITK Reviewer Group II, O1 MITK Reviewer Group I, s434n

Reviewed By: O3 MITK Reviewer Group II, O1 MITK Reviewer Group I, s434n

Maniphest Tasks: T29342

Differential Revision: https://phabricator.mitk.org/D736