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