Port to QWaylandWindow::setShellIntegration()

With QWaylandWindow::setShellIntegration(), it's possible to use
xdg-shell and layer-shell protocols in the same process. It's important
for plasmashell, where we want to use the layer shell protocol for
special surfaces such as the desktop background, and the xdg shell
protocol for dialogs.

In order to make a QWindow use the layer shell protocol, you need to
call LayerShellQt::Window::get() before the window is mapped.