basic support for border: each border need to be fully specified, no support for border-style, border-size or border-color

For instance, on <math> element: https://developer.mozilla.org/en-US/docs/Web/MathML/Element/math

resolves #190

Fixes a persisting issue from !98
  createBreakToken would not return the correct break token if not all content of the last element with data-ref have been rendered.
Fixes findOverflow who could not determine whether a node was displayed on the next column with page margin at 0
  due to a wrong comparison with floored left position and rounded end position. Now both are rounded.
Enhancement: findOverflow will skip children more frequently if right and end positions are equals.

fix issue when string was empty