ImageViewer: Fix and improve setting default attributes

[janezd]

Issue

Fixes #243.

Description of changes

Also change the models so that image attr must be a string, and title prefers strings (unless there is a class var, which has priority).

Includes

• [X] Code changes
• [X] Tests

[markotoplak]

@janezd, what is the state on this one?

[janezd]

@markotoplak, I think I addressed @VesnaT 's comments. Could you take a look at tests? If we allow them to fail, please merge and release.

[janezd]

I see we do, and I merged. Would you make a release?