[Kickoff] Anchor right of subTitleElement in KickoffItem to arrow.right

Summary: I meant to do this in D19874, but I must have accidentally reset the change.

Test Plan: Looks exactly the same as the screenshots in D19874, which were made with this change

Reviewers: #vdg, #plasma, filipf, ngraham

Reviewed By: #vdg, filipf, ngraham

Subscribers: filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20399