js: Add missing return values to later_add() handlers

js: Add missing return values to later_add() handlers

meta_later_add() is modelled after g_idle_add() and friends, and
the handler's boolean return value determines whether it should
be scheduled again or removed. There are some places where we omit
the return value, add them (although the implicit return value of
"undefined" already gives us the intended result).

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/637