Add translations for new memory sensors

Summary:
This adds the translated strings for the new Total, Available and
Allocated sensors.

Depends on D25305

Test Plan:
The sensor browser no longer shows the new sensors as "total", "available"
and "allocated" but "Total Memory" etc.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25306