Backspace/delete over non-removable elements will still move the cursor

Under some cases the selection will have a length, but will not contain
any removable elements (e.g., the cursor is either side of an annotation
). In order to have the cursor continue to flow naturally through the
document, the position must be reset to the new destination, so it will
appear to "move" even if no content is removed.

Addresses final issues in #56.