Simplify relationship object highlighting export mappings

[soininen]

Object dimension "highlighting" can be more easily implemented directly in RelationshipClassMapping and RelationshipMapping without the need for the special RelationshipClassObjectHighlightingMapping and RelationshipObjectHighlighlightingMapping. The new implementation also does not suffer from the scalability problems of the old.

Fixes #244

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

Patch coverage: 76.19% and project coverage change: -0.05 :warning:

Comparison is base (0e6c67b) 75.46% compared to head (4dadca1) 75.42%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #246 +/- ## ========================================== - Coverage 75.46% 75.42% -0.05% ========================================== Files 76 76 Lines 10895 10889 -6 Branches 2369 2374 +5 ========================================== - Hits 8222 8213 -9 - Misses 2234 2236 +2 - Partials 439 440 +1 ``` | [Impacted Files](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/246?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools) | Coverage Δ | | |---|---|---| | [spinedb\_api/export\_mapping/export\_mapping.py](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/246?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVkYl9hcGkvZXhwb3J0X21hcHBpbmcvZXhwb3J0X21hcHBpbmcucHk=) | `87.84% <75.00%> (-0.43%)` | :arrow_down: | | [spinedb\_api/export\_mapping/settings.py](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/246?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVkYl9hcGkvZXhwb3J0X21hcHBpbmcvc2V0dGluZ3MucHk=) | `77.31% <100.00%> (ø)` | |

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