[KFontChooser] Add a checkbox to toggle showing only monospaced fonts

Summary:
The use case is applications where the user is usually interested in
monospaced/fixe-width fonts, e.g. using Kate to edit source code.

Remove one redundant ++row.

Depends on D28271

Test Plan: See kfontchooserdialogtest app

Reviewers: Frameworks, dfaure, cfeck, apol, bport

Reviewed By: dfaure, bport

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28274