[ENH] Image Viewer - remember selection

[PrimozGodec]

Issue

The image Viewer widget does not remember the selection. It also fails when images are selected, and a smaller set of images is on the input so that the selection index is greater than the length of a new dataset.

Description of changes

Implement selection as the schema-only ContextSetting, which stores image names.

This PR also:

• changes that attributes are saved as Variables in ContextSettings (before indexes were saved - which doesn't make sense in combination with ContextSettings)
• removes the _noteCompleted method, which seems not to be used
• refactor the error to use the Error class instead of the error attribute.

Includes

• [X] Code changes
• [X] Tests
• [ ] Documentation

[codecov[bot]]

Codecov Report

Merging #230 (a1d8798) into master (faeabf7) will increase coverage by 2.90%. The diff coverage is 87.50%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #230 +/- ## ========================================== + Coverage 69.63% 72.53% +2.90% ========================================== Files 18 18 Lines 2832 2844 +12 Branches 393 395 +2 ========================================== + Hits 1972 2063 +91 + Misses 748 666 -82 - Partials 112 115 +3 ```

[PrimozGodec]

@janezd, I changed the selection setting to store image names. It is ready for review.