gantt: change sort method to class method (#7335)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11855 e93f8b46-1217-0410-a6f0-8f06a7374b81
gantt: add test to sort issues by current logic (#7335)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11856 e93f8b46-1217-0410-a6f0-8f06a7374b81