DolphinPart: port dolphin's last use of KRun::run to CommandLauncherJob.

Summary:
I chose CommandLauncherJob rather than ApplicationLauncherJob
because that would require either looking up org.kde.kfind.desktop
(with a risk of failure) or duplicating the icon name...

Test Plan:
commented out the if() block of the slot, to make sure we end
up launching kfind with this code. Then konqueror /, and Ctrl+F.

Reviewers: broulik

Reviewed By: broulik

Differential Revision: https://phabricator.kde.org/D28810