Handle Activities within a project like projects are handled from "root":
  Separate hash-ID directory for each activity
  Separate info-file with optional meta-data per activiy
  Separate tracking-file per activity.
  Activities stored as sub-directories of related project
  Keep track of current and last activity

Change functions/subs that handle projects currently to be
generic, and take PROJECT id as optional argument, following
the logic: if an argument is passed, the function should
operate on an activity, if no argument is passed, on project.

Fix code so start = stop->start can work
Make sure stop stops current activity, if any
Allow activities to toggle (activity = main->activity)

Be naive: assume that tracking files are consistent :)
Maybe: get inspiration from ti: read until last line, and regex new info onto it :)