[IconApplet] Port KRun to ApplicationLauncherJob

Test Plan:
Open the Application Launcher, right click any app -> Add to Panel (Widget),
then click the icon on the panel, it should launch.

Reviewers: #plasma, broulik, dfaure

Reviewed By: dfaure

Subscribers: dfaure, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29687