Kickoff: Fix grid view key nav in mirrored layout

KickoffGridView's keyboard navigation was broken when
LayoutMirroring.enabled=true, as could be seen by running
plasmoidviewer --applet org.kde.plasma.kickoff -reverse

This patch separates the concepts of left/right from line-start/line-end
and adjusts them as needed when layout mirroring is active.


(cherry picked from commit 733b05a6)