[SMS app] Mask regex special characters in search box

Summary: QSortFilterProxyModel filters based on a regex. When the user inserts regex special characters things go bad.

Reviewers: KDE Connect, apol

Reviewed By: KDE Connect, apol

Subscribers: apol, broulik, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D14642