Refix high CPU usage issue

After refactoring out Widget.needRedraw(),
the spans' equality check became malfunctioned
due to Generator comparision.
Changed Record.spans() to return a list instead of
a Generator to fix this.
Fix statistics toggle issue

Turn off statistics (press 's' again) is malfunctioned
before because self.update() is only called on toggle-on,
change to call self.update() both in toggle-on and toggle-off
to fix.