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.

includes/t-data/dao_items.inc.php

@@ -177,8 +177,9 @@ class DAO_Items
 		$query = $this->query(
 			'SELECT p.item_id , p.item_name , p.item_description , COUNT(*) AS t_count '
 			.	'FROM items p '
-			.		'INNER JOIN tasks t USING( item_id ) '
-			.		'LEFT OUTER JOIN completed_tasks c USING( task_id ) '
+			.		'INNER JOIN task_containers USING ( item_id ) '
+			.		'INNER JOIN tasks t USING( tc_id ) '
+			.		'LEFT OUTER JOIN completed_tasks c ON t.task_id = c.task_id '
 			.	'WHERE c.task_id IS NULL '
 			.	'GROUP BY item_id, p.item_name' );
 		$results = $query->execute( );