Fix positioning of the adder when the document or body elements are
positioned and offset relative to their default position.

There were two problems:

 1. When converting from viewport to "document" coordinates, code in
    `adder.js` and `range-util.js` failed to account for the document
    element's position being offset from the default (0, 0) location.
 2. When setting the adder's top/left coords, `Adder#showAt`
    did not take into account the offset of the body element from the
    document.

This commit fixes the issue by:

 1. Using viewport coordinates as far as possible in the range-util and
    Adder functions to reduce the need for converting coordinates.

 2. Calculating the position of the adder by comparing the target
    viewport coordinates for the adder and the viewport coordinates of
    the adder's nearest positioned ancestor.

Fixes #487