svghmi.c : deduplicated variable access code borrowed from plc_debug.c. Added targets/var_access.c.

SVGHMI: Added iterators in svghmi.c copy-pasted form plc_debug.c

SVGHMI: svghmi.c now has mutex, iterator, and read/write buffer.

SVGHMI: draft for svghmi.c. It has all PLC variables pointed in HMI tree in an array.

Pass HMITree to SVG transform. It seems it could really help to reduce JS tree binding logic in the end.

Add special nodes at HMI Tree root, fix code to handle special node (no path).

Swap CTN code gen and Libraries code gen, because SVGHMI use both, and need the library part to produce hmitree before CTN part.

yslt_noindent : Avoid YSLT's indent logic to interfere when using linequote.

SVGHMI: make a reasonably basic test

SVGHMI: filter out temporary variables created while generating ST code out of FBD.

SVGHMI: added deduction of HMI tree from list of HMI_* instances.

SVGHMI: added extraction of SVG bounding boxes, obtained from "inkscape -S", and passed to XSLT transform as variable.

Allow using docutil.get_inkscape_path() on Linux also.

SVGHMI: little fixes to make the build go through

SVGHMI: Added XSLT transformation, Makefile to get XSLT from ysl2 (copy of plcopen/Makefile) and a minimal stylesheet to start with.