Fix mass select items dialog, add "Select structural items" button

[soininen]

Mass select database items widget was missing "parameter_type" items, "metadata" should have been part of entity items, and "entity_alternative" part of scenario items.

Also, added a "Select structural items" button since it has been requested already quite some time ago.

Fixes #1077

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

All modified and coverable lines are covered by tests ✅

Project coverage is 66.96%. Comparing base (41790c3) to head (b0b14e2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2917      +/-   ##
==========================================
+ Coverage   66.94%   66.96%   +0.02%     
==========================================
  Files         189      189              
  Lines       36890    36901      +11     
  Branches     6314     6316       +2     
==========================================
+ Hits        24697    24712      +15     
+ Misses      10882    10878       -4     
  Partials     1311     1311              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.