New: pugixml doc structure for defs element

Code should not work here, but seems to be functional. <svg> closing tag occurs immediately after <defs> closing tag instead of at the end of the document.

This commit is mainly for a progress check.
Fixed: svg graphics are re-parsed and added to pugi structure

Followup commit to make the previous one work correctly. m_svg is re-parsed as XML and cloned to the svgDoc structure from before.
New: svg represented as pugixml objects instead of strings

-svgdevicecontext include file now contains node stack and current node

-nodeStack, xml declaration node, and main <svg> node are created in constructor of deviceContext. <svg> height and width are set in Commit as scaledHeight/Width are calculated later.

-All WriteLine code was replaced with pugixml equivalents.

Next commit will clean up any old fragments of code still remaining.