Allow associating additional metadata to a labelled set of stroke sequences

[mz2]

• An association to a vocabulary that the set of labelled stroke sequences is drawn from (e.g. "LaTeX math symbols", "Alphanumerical characters", "Proofreading markers").
• Symbol identifier (e.g. LaTeX command for the symbol).
• Human readable description.

[mz2]

Thinking more about this, probably the best way to associate the metadata is:

• Stroke sequence database has symbols identifiable by their label.
• Stroke sequence database has symbol namespaces set N.
• A symbol has a set of namespaces which is a subset of N.
• A stroke sequence has a symbol.

This way the same symbol can be part of multiple namespaces, and a stroke sequence can remain associated with just one symbol. This should be easy to build and still allows for building multiple levels of classification systems for the stroke sequences.