Make Appearance KCM tooltip texts consistent

Summary: Improve wording and add an ellipsis where needed in tooltips in Appearance KCMs.

Test Plan: Open for example the Plasma Theme KCM and hover over the "Edit Theme..." button.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: #vdg, plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19038