Clarify recommendations for font-specific metadata for fonts for text-based applications

[dspreadbury]

The specification doesn't make clear whether or not fonts for text-based applications should have font-specific metadata files, and it also provides no means for consuming applications to tell the difference between fonts designed for music notation applications and fonts designed for text-based applications.

We should both clarify the specification and also add a new key/value pair to the font-specific metadata file, perhaps something like targetApplicationType with an enumeration defining values like notation or text.