Call KRunner KRunner

Summary:
Plasma uses inconsistent terminology to refer to KRunner, calling it "Run Command" in
Kicker/Kickoff/etc, but "Plasma Search" in System Settings. For reasons explained in
{T10966} neither term is ideal, and the inconsistency is bad too.

This patch unifies on "Show KRunner" for the action and "KRunner" for the KCM name,
transforming it into a recognizable brand name like Apple's Spotlight.

This patch also makes the icons consistent and adjusts some strings to more clearly
indicate what KRunner is and does.

Test Plan:
{F6842587}

{F6842586}

Reviewers: #vdg, #plasma, ndavis

Reviewed By: #vdg, ndavis

Subscribers: ndavis, plasma-devel, kde-doc-english

Tags: #plasma, #documentation

Maniphest Tasks: T10966

Differential Revision: https://phabricator.kde.org/D21343