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

db/migrate/20100313132032_add_issues_nested_sets_columns.rb

@@ -0,0 +1,17 @@
+class AddIssuesNestedSetsColumns < ActiveRecord::Migration
+  def self.up
+    add_column :issues, :parent_id, :integer, :default => nil
+    add_column :issues, :root_id, :integer, :default => nil
+    add_column :issues, :lft, :integer, :default => nil
+    add_column :issues, :rgt, :integer, :default => nil
+    
+    Issue.update_all("parent_id = NULL, root_id = id, lft = 1, rgt = 2")
+  end
+
+  def self.down
+    remove_column :issues, :parent_id
+    remove_column :issues, :root_id
+    remove_column :issues, :lft
+    remove_column :issues, :rgt
+  end
+end