Make KHelpCenter DBusActivatable

@ltoscano

@davidre This was my ApplicationLauncherJob bug detector when it comes to url arguments 😄 gapplication launch org.kde.khelpcenter man:/gpg worked as expected, but

	const KService::Ptr helpCenter = KService::serviceByDesktopName(QStringLiteral("org.kde.khelpcenter"));
	auto job = new KIO::ApplicationLauncherJob(helpCenter);
	job->setUrls({QUrl(QStringLiteral("man:/gpg"))});
	job->start();

as called from kgpg did not :)