Correct a confusing variable name.
Clean up observers in `destroy`.

They were no longer being cleaned up after the removal of `onScroll`, and the
possibility that 'scroll.sticky' might be observed on _scrollArea_, not the
window.
Fix a regression where _duration_ wasn't used.

Introduced in 7df482d15b4b4dca22d702578502f880fd49263a.
Add an 'onReposition' callback.

To be invoked with the floated element when the element is repositioned.
Allow elements to be stuck to target elements rather their parents' visible bounds.
Always reposition the floated element when `update` is called.

This is useful when tracking a target element, to 'resync' the floated element and target
elements in cases where the target element may have moved for other reasons than
the scroll area scrolling or the window resizing.
Merge branch 'archive/correct_variable_name'
Merge branch 'archive/unobserve'
Merge branch 'archive/reenable_duration'
Merge branch 'archive/onReposition'
Merge branch 'archive/target'