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