Add a ToolTip role to PlasmaShellSurface

Summary:
As Wayland doesn't have a proper ToolTip window type yet, we add it
to PlasmaShellSurface, so that we can use it for Plasma's tooltips.

Reviewers: Plasma on Wayland

Subscribers: plasma-devel

Tags: Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D2030