Use Schema attributes for version info

[StokesMIDE]

In schemas, we currently use default values in some of the EBML child elements to define version info (DocType, DocVersion, ReadVersion, etc.). This is not the best place for it; the user has to root around to find the values when updating the schema, and it's a bit of a hack.

Some of the schemata have attributes in the root <Schema> element, like:

<Schema type="mide" version="2" readversion="2">

This is a better place for this sort of information, but it isn't actually used at this point. The schema parser should read these, and fall back to the <EBML> child element defaults if they aren't present (for backwards compatibility).

The default values of the EBML* child elements should also be elsewhere, either as additional <Schema> attributes, or maybe elements in <SchemaInfo> with the same name as the EBML element, like:

<?xml version="1.0" encoding="utf-8"?>
<Schema type="mide.ss.cmd" version="1" readversion="1">
    <SchemaInfo>
        <EBMLVersion value="1" />
        <EBMLReadVersion value="1" />
        ...

These are essentially constants, though, so they should be optional.