[Applet] Focus search field after showing it

Summary:
...Otherwise you need to manually click on it after showing it before you can start
typing into it, and that's just annoying.

BUG: 416482
FIXED-IN: 5.18.0

Test Plan:
Click search button to show search field
Search field is shown and has focus

Reviewers: #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26802