t/ckeditor5/1838: Used the new EditorUI#setEditableElement() API in the DecoupledEditorUI class

[oleq]

Suggested merge commit message (convention)

Internal: Used the new EditorUI#setEditableElement() API in the DecoupledEditorUI class (see ckeditor/ckeditor5#1838).

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A piece of ckeditor/ckeditor5#1840.

[coveralls]

Pull Request Test Coverage Report for Build 267

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 35 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-56.04%) to 43.956%

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Files with Coverage Reduction	New Missed Lines	%
packages/ckeditor5-editor-decoupled/src/decouplededitor.js	12	50.0%
packages/ckeditor5-editor-decoupled/src/decouplededitorui.js	23	29.27%

Totals
Change from base Build 265:	-56.04%
Covered Lines:	33
Relevant Lines:	68

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💛 - Coveralls