Use ActionTextField to implement the searchField in system settings

Summary:
Needs Kirigami latest version (master) and D19314

**Warning:** Remove the opacity change animation. :(

{F6637862}

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19315