The problem with KToolInvocation is that it creates a dead lock on
Wayland in case kdeinit is not already running. It starts kdeinit
and does a QProcess::waitForFinished and our kdeinit needs to interact
with the wayland server. So dead lock.

As KRun also calls into the dangerous code path it's no option which
leaves us with QProcess to start the processes.

A nice side-effect is that we no don't need to link KF5::Service any
more from kwin_core. Now once Plasma and Notification don't use it
any more, it will be gone completely.