Reduce information written to Designspace file

Currently, the amount of information written to Designspace files looks a bit much, with duplicated information, etc. See if we can go from e.g.

```XML

[...]

```

to

```XML

[...]

```

The prime obstacle will probably be the call to MutatorMath in fontmake, which uses old code to parse the Designspace file and may expect certain data.

Start by tracing how glyphsLib writes Designspace files, then inspect the MutatorMath parsing path used by fontmake, which the issue identifies as a compatibility risk. Compare the current output with the reduced examples and verify that the shortened files remain consumable by MutatorMath and fontmake.

Written by the indexing model from the issue text.