Call KRunner KRunner

Summary:
We are currentlly very inconsistent how we refer to KRunner, impairing comprehensibility
among both expert users (who know it as "KRunner") and novice users (for whom "Run
Command" has a literal and somewhat scary meaning).

This patch is a part of T10966: Unify how we refer to KRunner and standardizes on "Show KRunner", helping to turn
KRunner into a user-visible brand name like Apple's Spotlight.

Test Plan:

Reviewers: VDG, Plasma, ndavis

Reviewed By: VDG, ndavis

Subscribers: ndavis, plasma-devel

Tags: Plasma

Maniphest Tasks: T10966

Differential Revision: https://phabricator.kde.org/D21341