[kcmkwin] Add a clear button to the Desktop Effects search field

Summary: This patch adds a clear button to the Desktop Effects search field by using Kirigami's SearchField.

Test Plan: {F6973312}

Reviewers: #kwin, #vdg, davidedmundson, ngraham

Reviewed By: #kwin, #vdg, davidedmundson, ngraham

Subscribers: ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22286