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} and standardizes on "Show KRunner", helping to turn
KRunner into a user-visible brand name like Apple's Spotlight.

Test Plan:
{F6842497}

{F6842498}

Reviewers: #vdg, #plasma, ndavis

Reviewed By: #vdg, ndavis

Subscribers: ndavis, plasma-devel

Tags: #plasma

Maniphest Tasks: T10966

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