Use Kirigami.ActionTextField for the SearchField

Summary: Need Kirigami latest version (master)

Test Plan: A regression found (need D19300)

Reviewers: #discover_software_store, ngraham

Reviewed By: #discover_software_store, ngraham

Subscribers: ngraham, apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19301