Handle all translatable columns

[jnm]

In addition to

formpack/src/formpack/utils/expand_content.py

Line 178 in 29bd37f

if column_name in ['label', 'hint']:

…we need constraint_message, required_message, and guidance_hint to be recognized even if they don't have a ::language suffix. Check http://xlsform.org/ to make sure we aren't missing any other columns.

The list of columns should be stored in an easy-to-find constant so that it can be kept up-to-date as the XLSForm specification evolves.

[jnm]

Can we get a list of translatable columns from pyxform? (Alex's idea)

[joshuaberetta]

This seems to be where pyxform handles the various translatable columns — unfortunately not great for integrating into formpack:

https://github.com/XLSForm/pyxform/blob/cf7ba39d4c7214c3ff7f43394fb057622fd21382/pyxform/survey_element.py#L275-L279