xwayland: change rooted xwayland from deprecated wl_shell to stable xdg-shell protocol

Recently, rooted Xwayland crashes on wlroots-based compositors, because wlroots removed the deprecated wl_shell protocol. This MR fixes this by changing the code in question to the xdg-shell protocol. My motivation do this: on etnaviv-based embedded platforms, rooted Xwayland is much faster and doesn't cause UI rendering bugs compared to rootless Xwayland.