Make Design view non-closable and non-floatable

[soininen]

Design view's dock is now non-closable and non-floatable.

Resolves #2963

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 67.06%. Comparing base (bdaac28) to head (cf275a7).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2981   +/-   ##
=======================================
  Coverage   67.06%   67.06%           
=======================================
  Files         190      190           
  Lines       37120    37121    +1     
  Branches     5327     5327           
=======================================
+ Hits        24894    24897    +3     
+ Misses      10964    10963    -1     
+ Partials     1262     1261    -1     

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