xwayland: port rooted xwayland from wl_shell to 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.

Signed-off-by: Lukas F. Hartmann <lukas@mntre.com>
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