Make SMS sending via KDE Connect work

Summary:
Similar approach as in the second iteration of D20353, and no longer
special-casing KDE Connect but relying on system-wide URL handlers.

Needs https://invent.kde.org/kde/kdeconnect-kde/merge_requests/50 to work.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D20447