New methods to add to a font via an extension

[dpvc]

This PR adds methods to support creating an extension that modifies an existing font. For example, an extension could override the characters used for the letters and numbers while leaving all the other characters in place, or could add coverage for special symbols that are missing from the font originally.

There are two new addExtension() methods: a static one on the FontData class that modifies the class defaults before it is instantiated, and one on the FontData instance that modifies the data for a given font instance. This allows a font to be extended before it is used, or to be extended dynamically once it is in place.

The font tools (currently under construction) will create the needed data and extension files that call the new addExtension() methods.