The Task Manager user interface increments the refcounts of the
tasks which it renders, but never unrefs them when the dialog is
closed.

A separate but related bug is that when rows are moved, they pick
up an additional unbalanced refcount.

In both cases, the refcounts should be balanced.

This bug appears to have been introduced when we moved to
GtkCTreeView for the task manager, and smoked out by accident
late last week when I moved the acache_checkin in task-save
from decode.c to task_save_destructor().  Since the latter
was never called, my cache file was filling up with hundreds
of files... :)