Random improvements to DB editor UX

[soininen]

• The search bar editor used e.g. in Parameter value table now matches the search string to any position, not just the beginning. Parameters should now be a lot more discoverable as you can just type in 'cost' to get all parameters with 'cost' in their name.
• The down key now adds a new row to some tables in DB editor making it faster to e.g. add new entities using the keyboard alone.

No associated issue.

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 & isort
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 8 lines in your changes missing coverage. Please review.

Project coverage is 67.22%. Comparing base (4d5999a) to head (afefed7).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
spinetoolbox/widgets/custom_qtableview.py	33.33%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3047      +/-   ##
==========================================
- Coverage   67.24%   67.22%   -0.03%     
==========================================
  Files         194      194              
  Lines       37274    37284      +10     
  Branches     4472     4475       +3     
==========================================
- Hits        25064    25063       -1     
- Misses      10939    10950      +11     
  Partials     1271     1271              

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