[KCharSelect] Initially give focus to the search lineedit

Summary:
If the search lineedit widget is added, give it initial focus, so that you
launch the dialogue and start typing to find the character you want;
(much like a web browser).

Test Plan: make && ctest

Reviewers: Frameworks, cfeck

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29362