Use a faster way to determine kio-stash isn't installed

Summary:
Dolphin needs to figure out whether or not kio-stash is installed
to set the enabled state of a UI action.

When KProtocolInfo::isKnownProtocol can't find a protocol it gets
worried its protocol cache might be out of date, so it rebuilds it
(doing plenty of disk I/O) and looks again. kio-stash is currently
not yet installed on many systems, so this means most Dolphin
startups out there.

This patch switches to using QDBusConnectionInterface::isServiceRegistered
instead to determine whether the stash notifier daemon is running,
which should be faster than doing disk I/O.

Reviewers: davidedmundson, elvisangelaccio, shortstheory

Subscribers: emmanuelp, alexeymin, ngraham, #dolphin

Differential Revision: https://phabricator.kde.org/D9094