Implement more of the SCons build

SConsProject/Config.py:
SConsProject/__init.py__:
- Add project-specific config file.  I'm not sure this is how it will really end up being organised
but this will do for now.

SConstruct:
- Add some separator comment lines to tidy this file up.
- Pull in project-specific config.
- Move definition of charms_yml and charms_yml_in_build earlier,
so I can set up dependencies on those files.
- Fix suffixes for DotToPng builder, so it actually works.
- Add AscToHtml builder, with dependencies on PNGs.
- Add YamlToDracHtml builder, which also copies over dracula JS library scripts.
- Add builder calls for all charm trees, for Yaml2Dot, YamlToAsc, DotToPng,
AscToHtml, YamlToSvg, YamlToDracHtml.  (Without the builder calls,
SCons knows *how* to build classes of files, but doesn't know that it *should*
build specific files.)
- Add builder for overall "quick HTML for Charms" output.

tools/dracula/* -> src/text/scripts/dracula/*:
- Moved these files under "src", so SCons can copy them, so the output is self-contained.

yaml2dracula.rb:
- Changed location of dracula script files.