Replace sort key lambdas by attribute and item getters where applicable

[soininen]

No functional changes intended. The attribute and item getters provide small performance boost over lambdas.

Re Spine-project/Spine-Toolbox#1854

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