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

app/views/issues/_form.rhtml

@@ -5,6 +5,16 @@
                                      :with => "Form.serialize('issue-form')" %>
 
 <p><%= f.text_field :subject, :size => 80, :required => true %></p>
+                                     
+<% unless (@issue.new_record? && @issue.parent_issue_id.nil?) || !User.current.allowed_to?(:manage_subtasks, @project) %>
+<p><%= f.text_field :parent_issue_id, :size => 10 %></p>
+<div id="parent_issue_candidates" class="autocomplete"></div>
+<%= javascript_tag "observeParentIssueField('#{url_for(:controller => :issues,
+                                                       :action => :auto_complete,
+                                                       :id => @issue,
+                                                       :project_id => @project) }')" %>
+<% end %>
+
 <p><%= f.text_area :description,
                    :cols => 60,
                    :rows => (@issue.description.blank? ? 10 : [[10, @issue.description.length / 50].max, 100].min),