allow intent and arg attributes in schema and pres MathML output

[dginev]

This is a minimal changeset that enables bindings to deposit intent and arg attributes on XMath nodes, making experimentation possible outside of mainline grammar-based XMath generation.

Two pieces to the PR:

• allow intent and arg attributes in the schema
• propagate intent in the top-level presentation node and intent and arg in all inner nodes.

This can certainly be developed further, but it is really helpful as a starting point - otherwise there is no way to persist a deposited intent attribute through post-processing.