[client] Add methods to get ShellSurface from a QWindow

Summary:
This is similar to what already exists for Surface. With this methods
it's possible to get a ShellSurface from an existing QWindow and to
perform low level native calls directly for the ShellSurface.

Similar calls will also be needed for XdgShellSurface, though the
xdg_shell_surface is not yet available through the native interface.

Test Plan: Used in breeze to trigger window moving

Reviewers: Plasma on Wayland

Subscribers: plasma-devel

Tags: Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D3018