Support media labels for selectOne and selectMany

[florianm]

Similar to #244 it would be great to have media labels available for option labels.

Since multiple media labels seem to work (at least as question labels), a use case would be a form "bird survey" showing both the (clickable big-)image of each species, plus offering an audio clip of their call, plus offering a video clip of their flight.

The media labels would probably fit well enough into the bulk edit popup, but might be a bit much for the sidebar. Maybe there we'd just add a hint for the properties sidebar "enter media label filenames through bulk edit".

Happy to attempt a PR after #244 is merged.

Related: #150 could save presets through import/export as CSV. Having media labels (plus their translations) will certainly post a sizing challenge to the Build choices modal, but could be far easier edited in an external spreadsheet. (Shush, XLSForm, stop snickering)