title: Eclipse Task List - Mark Completed is not Active
body: <p>The Title says it all. I am using ADT and I have a source code full of <code>//TODO</code> tags.
  I can go through the tags using the <em>Tasks View</em> but when I want to mark a task as completed the <code>Mark Completed</code> option is grayed out. <br>
  what is the cause of this and how can I fix it ? deleting the <code>//TODO</code> tags for completed tasks is not an option, I want to be able to review my changes later.</p>