Always draw the tooltip on top of everything else

Summary:
Without this, the tooltip would be drawn below things that manipulate z. With
it, the tooltip is always drawn on top

Test Plan: Tested with adding a toltip to this Kamoso patch: D18170

Reviewers: hein, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18195