[tei-council] change/@date should be change/@when, no?

[Syd Bauman]

I'm thinking that the date= attribute of the <change> element should
be renamed to when=. This seems better for 2 reasons:

* matches when= of <date>, <time>, and all other datable elements;

* the value may be a time, or a combined date & time, as opposed to
  just a date.

This seems uncontroversial enough that Unless there are objections
I'll plan to do this (and mimeType=) tomorrow.