Improve Commit viewer

[soininen]

Commit viewer can now handle large databases as well.

• The Affected items tree has been replaced by tabs, each tab corresponding to different item type and containing a table of affected items.
• Affected items are now fetched in a worker thread to keep the Viewer responsive.
• We now fetch maximum of 400-500 affected items and show the number of unfetched items in a text label. It does not seem to make sense to load thousands of items into the table.
• Other small usability improvements.
• The item_type and id_ parameters of DatabaseManager.get_value() have been replaced by item parameter which makes the method faster when the item is already at hand (the benchmark runs 1.9x faster).

Fixes #2602

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 81.78694% with 53 lines in your changes are missing coverage. Please review.

Project coverage is 65.70%. Comparing base (17da6a2) to head (eec1d8c).
Report is 1 commits behind head on 0.8-dev.

Files	Patch %	Lines
...netoolbox/spine_db_editor/widgets/commit_viewer.py	68.78%	41 Missing and 8 partials ⚠️
...e_db_editor/ui/commit_viewer_affected_item_info.py	95.45%	0 Missing and 1 partial ⚠️
...pinetoolbox/spine_db_editor/ui/db_commit_viewer.py	98.91%	0 Missing and 1 partial ⚠️
...inetoolbox/spine_db_editor/widgets/custom_menus.py	0.00%	1 Missing ⚠️
spinetoolbox/spine_db_manager.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           0.8-dev    #2712      +/-   ##
===========================================
+ Coverage    65.52%   65.70%   +0.17%     
===========================================
  Files          194      196       +2     
  Lines        35674    35866     +192     
  Branches      6066     6090      +24     
===========================================
+ Hits         23376    23565     +189     
+ Misses       11049    11042       -7     
- Partials      1249     1259      +10     

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