Allow use of field value to populate alt text

[fbcideas_migusr]

Currently, alt text on any visual is populated by static text entered by the report designer. It would be more helpful to be able to use a field value or do something like the Enlighten Data Story where you can put a placeholder for multiple field values. Ideally, the alt text on a visual provides a summary of what you should glean from the visual, which could change as data is refreshed. So we need a way to dynamically populate the alt text property.