changes:
Progress bars will now stack if there is multiple ones on a single turf.
Progress bars' update speed is now mediated by stoplag() instead of arbitrary numticks numbers.
A callback can now be passed to do_after or do_mob which will be called every check - if the callback returns FALSE, the action is cancelled.
Progress bars no longer show an X briefly when their associated action is cancelled.
Progress bar itself is from /tg/, do_after changes are not.
