Support solidColor element from SVG 2

[igagis]

Add solidColor element. Implement parsing from XML and writing to XML. Add test.

This element was introduced in SVG spec version 2. While this library aims to support SVG 1 for now, this element might be useful. https://www.w3.org/TR/2013/WD-SVG2-20130409/pservers.html#SolidColors

Potential use case is to be able to change colors in SVG dynamically.