Initial sub-tasks support

In addition to normal dependencies, the application now supports
sub-tasks. Sub-tasks can be added into any existing task (including
other sub-tasks, Inception-style).

Dependencies can only be added between global tasks, or between
sub-tasks of the same task. It is no longer possible to mark a task as
finished if it has incomplete sub-tasks, and conversedly, it is not
possible to reactivate a sub-task if its parent is marked as completed.

A pair of buttons allowing tasks to be moved up and down in the task
hierarachy have been added.

database.sql

@@ -14,8 +14,10 @@ BEGIN;
 \i database/items-functions.sql
 
 -- Task management and task dependencies
+\i database/task-containers.sql
 \i database/tasks-functions.sql
 \i database/task-dependencies.sql
+\i database/tasks-move-sub.sql
 \i database/tasks-view.sql
 
 COMMIT;