[applets/appmenu] Set iconSource properly

Summary: setting an icon via i18n makes no sense

Test Plan: Put in vertical panel, had an icon

Reviewers: #plasma, broulik, ngraham

Reviewed By: #plasma, broulik, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18480