Merge branch 'hotfix/#6891-#6778-composer-json-string-escaping'

Close #6891
Reproducing test case for #6263 #6518
Workaround for #6263, which is related to #4482
Additional unit test to assert behavior of extract()
Refactor Collection::extract to reduce branch depth & add documentation
Merge branch 'hotfix/#6880-form-element-can-be-added-to-a-fieldset' into develop

Close #6880
Close #6518
Close #6263
Forward port #6880
Forward port #6518
Forward port #6263