Improve partial maximization strings

Summary:
e.g. "Vertically maximize" is more natural and expressive than "Maximize (vertical only)"

See D29671#inline-169882.

Test Plan: New strings appear in the KCM

Reviewers: zzag, VDG, KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D29729