set skipTaskbar on SurfaceEvent::SurfaceCreated

Summary: makes sure the flag is set on time

Test Plan: can't get the panelcontroller in the taskbar anymore

Reviewers: #plasma, hein, broulik

Reviewed By: #plasma, hein, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18186