s/m_xdgShellSurface/m_xdgShellToplevel/g

Summary: m_xdgShellSurface represents an xdg-toplevel, not an xdg-surface.

Test Plan: Compiles.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26975