Focus search field by default

Summary:
In general, apps that have Search Fields always visible put the focus there by default.
For example: System Settings and Discover. However KMenuEdit does not follow this pattern.

This patch fixes that.

BUG: 413563
FIXED-IN: 5.18.0

Test Plan: Open kmenuedit and start typing because the search field is now focused by default

Reviewers: VDG, Plasma, cfeck

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25145