Select with single-language labels and choice filter can't specify choice images

[lognaturel]

Software and hardware versions

all

Problem description

Images in ODK XForms are displayed using the itext machinery. When a select has labels in a single language and a choice filter, the choices are represented as a secondary instance and the labels are taken from the label element for each item in the instance. In other words, itext isn't used. This means there's no way for images to be displayed by clients.

Steps to reproduce the problem

Create an XLSForm with a select that has single-language labels and a choice filter such as this one.

Expected behavior

It should be possible to specify images for selects with single-language labels.

Other information

If there's media specified for choices, itext should always be used for specifying the choice labels. #463 is in the same general space.