[BZ#760191] Add support for baseline-shift=<percentage> and <length>
Submitted by Federico Mena Quintero
Bug #340047 added support for the baseline-shift attribute for text objects; it supports "sub", "super", and plain "baseline" with hardcoded values. We still need to add support for percentages of the text size and for explicit lengths.
This is http://www.w3.org/TR/SVG/text.html#BaselineShiftProperty in the spec.