Accept different YAML types for image(s)

[kvid]

• Accept a single string - fixes [feature] Accept a single string as image value #292
• Accept a list of images - fixes Several pictures for one connector ? #407
• Accept no image (as today) - also consider empty string/list as none
• Accept dict (as today)
• Raise TypeError for unexpected input types
• TODO: The current implementation of resolving image paths would require checking all the different input types twice: 1) in the parse() function to find and resolve the paths (not yet fully implemented), and 2) in Image.create() to create the instance(s). How can this be simplified by e.g. forwarding the context (with image path list) to Image.create() to avoid duplicating all the type testing?
• TODO: This PR is currently based on master to simplify testing the concept, but will later be rebased onto dev and the single commit about HTML generation adapted to Large scale refactoring #251.