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 wlr_xdg_popup.parent.data,
create a new scene-graph node via wlr_scene_xdg_popup_tree_create,
and unconstraint the popup if they want to.