text improvements for Colors, Icons, Style, and Workspace KCMs

Summary:
BUG: 386570
BUG: 386565
BUG: 386575
BUG: 386567

Implement some of @abetts' proposals in the above-mentioned bugs

Test Plan:
Tested in KDE Neon:

{F5479678}

{F5479394}

{F5479213}

{F5479363}

Reviewers: abetts, #vdg, #plasma, #documentation, apol

Reviewed By: abetts, #vdg, #plasma, apol

Subscribers: lueck, apol, plasma-devel, abetts

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8684