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-items/controllers.inc.php

@@ -280,6 +280,6 @@ class Ctrl_ItemTasks
 		return Loader::View( 'box' , 'Tasks' , Loader::View( 'tasks_list' , $tasks , array(
 					'deps' , 'assigned' , 'completed' ) ) )
 				->addButton( BoxButton::create( 'Add task' , 'tasks/add?to=' . $this->item->id )
-				->setClass( 'list-add' ) );
+					->setClass( 'list-add' ) );
 	}
 }