Represent if the task is actionable in UI

[izidormatusov]

There are many rules that decide if the task shows up in workview:

    def workview(self, task, parameters=None):
        wv = (self.active(task) and
              self.is_started(task) and
              self.is_workable(task) and
              self.no_disabled_tag(task) and
              task.get_due_date() != Date.someday())
        return wv

Provide an icon for in the task editor toolbar that will have 3 states:

• task is doable: something positive like a check mark
• task is done or dismissed: hide the icon
• otherwise: display something less positive, have a tooltip that shows reasons behind it:

- Task has unfinished subtask: {subtask_title}
- Task doesn't have a due date, is due somday
- Task have disabled tag {tag}
- Task will start on MM/DD

Please, come up with a better wording.