[Kickoff/Dash] Show Calculator and Unit Converter runners

Summary:
The Calculator and Unit Converter runners are some of the most useful and
productivity-boosting runners, yet they don't show up in Kicker/Kickoff/Dash
right now, only in KRunner.

This patch makes them show up in Kickoff and Dash, increasing discoverability
and productivity for regular users who are less likely to know about KRunner's
standalone UI.

Kicker is excluded because its search UI was not designed for multiple runners
and that needs to be fixed first (https://bugs.kde.org/show_bug.cgi?id=389146)

CCBUG: 382760

Test Plan:

Reviewers: VDG, Plasma, hein, filipf

Reviewed By: VDG, Plasma, hein, filipf

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21983