* Initial work on refactoring the font metric code

- Moved away from the archaic method of writing out PHP to a file and including it in the next run, by writing out the metrics data as JSON and reading from the file as required.
- General tidying up of code and trying to make sense of the > character 127 processing, and why we're doing what we're doing.
- For some reason, character widths appear to be broken for characters > 127 when reading the 127 file - when the file doesn't exist and we process on-the-fly, the widths appear to be OK. Hmmz.

* A bunch of reactors and cleanup around writing and accessing the font metrics and character widths files