Introduce wlr_xdg_surface_for_each_popup

Created by: RyanDwyer

It is common to want to iterate an xdg-surface's popups separately from the toplevel and subsurfaces. For example, popups are typically rendered on top of most other surfaces.

wlr_xdg_surface_for_each_surface continues to iterate both subsurfaces and popups to maintain backwards compatibility, though it now iterates all the popups after all the subsurfaces rather than mixing them. Changing this back requires several lines of duplicated code, and I figured it's not likely to cause any issues. This part has been reverted.