Don't use kcmshell5 for opening the KCM

Summary:
kcmshell5 depends on plasma-workspace. By using our own tiny wrapper for opening the KCM we avoid pulling
in plasma-workspace and everything that comes with it on non-Plasma systems.

Test Plan: kdeconnect-settings shows the KCM

Reviewers: KDE Connect, sredman

Reviewed By: KDE Connect, sredman

Subscribers: sredman, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D17383