Before this commit, when the user changes the stage of a task linked to
a milestone, he cannot know if after that task, there is another one
before reaching the milestone related.
This commit displays a wizard to allow the current user to directly
mark as reached the milestone when the user changes the stage of a
task linked to a milestone to a closed stage and all tasks linked to
that milestone are done too. However, if the milestone is already
reached, no wizard will be displayed.
task-2829542