scene: add wlr_scene_xdg_surface_create

Merged Simon Ser requested to merge github/fork/emersion/scene-xdg-popup into master

This allows compositors to easily add an xdg_popup to the scene-graph while retaining the ability to unconstraint the popup and decide its final position.

Compositors can handle new popups with the wlr_xdg_shell.new_surface event, get the parent scene-graph node via, create a new scene-graph node via wlr_scene_xdg_popup_tree_create, and unconstraint the popup if they want to.

Edited by Simon Ser

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