Make symbol library for use by Inkscape

[jaredjennings]

Here is an XSLT stylesheet and accompanying PowerShell script that gather the individual SVG files into a single SVG usable as an Inkscape symbol library. In a separate commit I've added the built symbol library.

It would be quite easy to perform the same gathering transformation using other languages or platforms (e.g., POSIX sh + xsltproc) to perform the gathering work on other platforms. But Windows is what I have handy right now, so PowerShell is what I've written here so far.