Allow Image to optinally have alt text specified

Merged snippet requested to merge topic/3.2/image_alt_text into branch/3.2

Optionally allow Image(img) to be called with Image(img, 'this is alt text') which would set 'this alt text' on an image. Adds a desc property to the Image class which stores the description. Should close #1850 (closed)

This is the rebasing of !417 (closed)

--HG-- branch : 3.2

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