Automatically unlink stack if control points are present inside the stack

[Janne]

Since control points are used to align images it's kind of counter-intuitive to have stacks linked when/if there's control points defined between the images of the stack. More than once I've had to restitch everything because the stacks were still linked.

Also there's special case when there's one defined stack with control points and the optimizer does nothing if optimizing image positions (y,p,r) only.

Screenie: http://i.imgur.com/v6E73.png