This commit changes the behavior of `new_*` signals to better match
their names (see https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3608).
wlr_xdg_shell.events.new_surface is now emitted when an xdg_surface is
created, and wlr_xdg_shell.events.new_{toplevel,popup} events are
introduced to get notified when an xdg_{toplevel,popup} is created.
Same applies to
`wlr_xdg_decoration_manager_v1.events.new_toplevel_decoration`. As a
result, wlr_xdg_surface.added and wlr_xdg_toplevel_decoration_v1.added
are removed, as we no longer need to track whether the corresponding
event was emitted.
Additionally, this commit changes the behavior of
wlr_xdg_surface.events.destroy: it is now emitted when the xdg_surface
is destroyed, as the name suggests.
wlr_xdg_{toplevel,popup}.events.destroy events are added to get
notified when an xdg_{toplevel,popup} is destroyed.
c85838892d