Add test showing position stability issue near annotations
Add additional tests showing position stability problems
Fix position instability around highlighted annotations

By checking only nodes that pass the supplied node filter, the position
logic can safely ignore out-of-place cursors near highlighted annotations.

Note, this can't re-use PositionIterator.previousNode, because the logic
requires the previous sibling of the current container's previous sibling.
Merge pull request #824 from peitschie/position-stability

Fix position instability around highlighted annotation