Changed behavior for valid_tracker_list() and is_valid_tracker() validator : they should NOT do anything if the module is not enabled. Also, when building a new issue (no ID allocated), check properly if we at least have a valid usable tracker registered before returning an error (impending rewrite of the whole method because of custom attributes handling)
Fixed wrong permission labels for trackers in the "Roles and permissions" edit page.

* "Create tracker<id>" was shown instead of "Create <name_of_tracker> tracker"
+ Added the TrackerHelper module to encapsulate tracker permissions symbols naming to improve maintainability
Merge remote-tracking branch 'base/redmine2' into redmine2