Support guidance and short label

[florianm]

This issue complements https://github.com/getodk/build/pull/244 Merging #12 and #14 first would provide a clean starting point.

Guidance

XLSForm supports guidance as guidance_hint as per docs.

The following steps work and produce the attached file Short-guidance-export.xlsx

• Add guidance_hint to survey-fields https://github.com/getodk/build2xlsform/blob/master/src/convert.ls#L15
• Add guidance_hint to multilingual-fields https://github.com/getodk/build2xlsform/blob/master/src/convert.ls#L18
• Add guidance: \guidance_hint to https://github.com/getodk/build2xlsform/blob/master/src/convert.ls#L21

Short label

The XLSForm spec does not mention a short label anywhere. Is a short label supported at all? If it's not supported, then maybe issue a warning about information loss like at https://github.com/getodk/build2xlsform/blob/master/src/convert.ls#L238 ?

[lognaturel]

Is a short label supported at all?

It's not. Tracked at https://github.com/XLSForm/pyxform/issues/34.

[florianm]

Closing this, as guidance hint is now supported.