Use concrete object types in side Panel

[didoda]

This provides a refactor to use concrete object types in object view side panel, when you open it (on "add objects" button click, for a relation, for example).

This also updates placeholders plugin to provide proper "available types" when you try to add placeholder and side panel opens.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7769f7a) 99.67% compared to head (6effabd) 99.67%.
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1077   +/-   ##
=========================================
  Coverage     99.67%   99.67%           
  Complexity     1166     1166           
=========================================
  Files            91       91           
  Lines          4601     4603    +2     
=========================================
+ Hits           4586     4588    +2     
  Misses           15       15           

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