Remove expensive blocking call on plasmashell startup

Summary:
QDBusInterface is a blocking call that calls introspect. We don't need
to introspect.

As this is called on startup this call starts kuiserver, which means
plasma is blocked whilst a whole process spawns, this is a lot worse
than just a regular, ~100ms on my system

Also using the generated versions is generally neater to read.

Test Plan:
Restarted plasmashell
Copied a file
Still had a spinny wheel
Q_DBUS_BLOCKING_CALL_MAIN_THREAD_WARNING_MS=0 doesn't list kuiserver

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15815