Context handler that respects position in Domain

[markotoplak]

Issue

Resolves #6721

Description of changes

Added a new context handler that respects context settings' attributes exclude_attributes, exclude_metas and exclude_class_vars. This context handler avoids storing the whole domain into settings and thus perfect matches do not work.

I searched for possible instances of #6721 in this repo and found only 3 cases where a limited DomainModel was used for a context settings, out of which only 1 was problematic:

1. owcorrelations, feature_model: not a problem, because it is applied to a processed data set where every possible feature is moved into .attributes.
2. owtestandscore, feature_model (for CV by Feature): not the same problem, because it uses PerfectDomainHandler.
3. owheatmap, row_side_color_model: finally, a bug. It this case, because of heavy customization, there were no crashes but UI was in an inconsistent state.

In Heat Map, a combo box for choosing row annotation color is limited to metas and class vars, and thus, when a selected feature was moved from metas to attributes, an incompatible context would match. That did not trigger any crashes because the combo box was updated through special functions, but it did leave the UI in an inconsistent state: an element with index of -1 was selected in the combo (""), and, the saved color was still shown in the map. Fixed by using the proposed ContextHandler.

Includes

• Code changes
• Tests
• Documentation

[markotoplak]

I tried adding dummy .attributes and .metas to the Context, hoping that new saved contexts would also work on older versions. Testing showed this does not work: (new) saved settings can get overridden by an (old) saved perfect match.

So I can't make these compatible, at least not that simple.