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/roles.yml

@@ -14,6 +14,7 @@ roles_001:
     - :add_issues
     - :edit_issues
     - :manage_issue_relations
+    - :manage_subtasks
     - :add_issue_notes
     - :move_issues
     - :delete_issues
@@ -66,6 +67,7 @@ roles_002:
     - :add_issues
     - :edit_issues
     - :manage_issue_relations
+    - :manage_subtasks
     - :add_issue_notes
     - :move_issues
     - :delete_issues