[designspace] Allow axes to be identified by tag as well as by name

[Lorp]

It would be convenient, especially when hacking XML .designspace documents, to be able to refer to an axis by its 4-character tag (e.g. "wdth") rather than its human-readable name (e.g. "Width"). Note that, in practice, axis tags are less likely to change during development than axis names, so if there is a name change, most of the XML can remain unchanged. It also makes code easier to reuse and to write without errors, since axis tags are standardized but axis names are not.

This would apply to (at least):

• <sources><source><location><dimension tag="????">
• <instances><instance><location><dimension tag="????">
• <rules><rule><conditionset><condition tag="????">
• <mappings><mapping><input><dimension tag="????">

I do not propose that support for axis names is removed.

[behdad]

The cleanest way to do this, such that the Python API is not changed, would be to convert the tag to name in the Python object...

[justvanrossum]

In retrospect, perhaps the tag should have been the identifier. I think it's too late to change now, and allowing both will cause more confusion than it solves. So -1 from me.

[Lorp]

Related: if two axes have identical tags, must they have identical names?