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

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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