Adds subtasking (#443) including:

* priority, start/due dates, progress, estimate, spent time roll-up to parent issues
* descendant issues tree displayed on the issue view with context menu support
* issue tree display on the gantt chart
* issue tree copy on project copy
* unlimited nesting

Defining subtasks requires the new permission 'Manage subtasks'.
Subtasks can not belong to a different project than the parent task.

Implementation is based on scoped nested sets for fast reads and updates.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3573 e93f8b46-1217-0410-a6f0-8f06a7374b81

test/fixtures/enumerations.yml

@@ -19,27 +19,32 @@ enumerations_004:
   id: 4
   type: IssuePriority
   active: true
+  position: 1
 enumerations_005: 
   name: Normal
   id: 5
   type: IssuePriority
   is_default: true
   active: true
+  position: 2
 enumerations_006: 
   name: High
   id: 6
   type: IssuePriority
   active: true
+  position: 3
 enumerations_007: 
   name: Urgent
   id: 7
   type: IssuePriority
   active: true
+  position: 4
 enumerations_008: 
   name: Immediate
   id: 8
   type: IssuePriority
   active: true
+  position: 5
 enumerations_009: 
   name: Design
   id: 9