Added the Command Line Interface (src/cli.py)
Not describing it right now, since I shall write the help-text soon anyway.

Additionally:
(1) Tasks are now created with references to their original groups.
(2) The logic for field generation for tabulation shifted to the Task class.
(3) Added TaskFile.update(group).
	Note: Had added TaskFile.delete(group) before realizing that it was
	kinda pointless. The same effect can be achieved by using update with an
	empty taskgroup.
(4) TaskGroup instance store the names of the actual groups in the file from
	which they were derived (or empty strings otherwise).
(5) TaskGroup.task_list() prefers not to return #hidden tasks at all. But
	that does not mean they aren't stored in the group.
(6) Renamed Task*.*_delete() to Task*.*_remove() and added Task.tag_check()
Deleted src/__watch.py because it wasn't really necessary.

It is useful in case of a web-server where we are no longer forced to manually
restart before reloading the page. However, that is not the case here.