Proposal: Introduce formalize metadata attribute + format

[bernardkwok]

This is a proposal to start discussion to formalize metadata tagging at the node definition level.
The proposal currently has two parts:

1. Support for a formal name tag / attribute name for metadata. e.g. metadata
2. Support structured data which may be in multiple languages in a standardized format that is easy to parse and write to.
   Perhaps something like XLIFF (https://en.wikipedia.org/wiki/XLIFF).

• The multi-language support is important to allow for data in arbtirary languages to be embedded.
• If there are "standard" templates for metadata then they could be reused.
• If not then structure data at least allows for a formal mechanism to group meta data that has some commonality.
• For migration some existing data could already be in this or a compatible format (which could be mapped).

An additional property is that if metadata is added then it can be referenced and placed either in a separate location or
be placed in a separate section within the same document for readability. This would imply a metadata type / definition.