To support scheduling performance-measurement scripts that want to run
a number of actions in series, add shell_global_run_at_leisure() to run
a callback when all work is finished.
The initial implementation of this is not that accurate: we track
business in Tweener.js via new shell_global_begin_work(),
shell_global_end_work() functions, and we also handle the case
where the main loop is continually busy.
https://bugzilla.gnome.org/show_bug.cgi?id=618189
a9a513c621