Update Placeholder after initialization

[illia-stv]

Suggested merge commit message (convention)

MINOR BREAKING CHANGE (engine): placeholder could be be updated after initialization. Closes #9925.

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Additional information

For example – encountered issues, assumptions you had to make, other affected tickets, etc.

[mlewand]

Couple of remarks, there was also no breaking change information - be sure to add it as this is a backward-incompatible change.

Also, I don't see a PR in collaboration features repo. You must also add a related PR in there as at least commenteditor.ts file uses the enablePlaceholder function.

[mlewand]

The following docs are no longer true:

ckeditor5/packages/ckeditor5-engine/src/view/placeholder.ts

Line 27 in 54a683f

* To change the placeholder text, simply call this method again with new options.

You should update it so that it describes how one should change the placeholder from here on.

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ckeditor5/packages/ckeditor5-editor-balloon/src/ballooneditorui.ts

Line 106 in ce44c03

* Enable the placeholder text on the editing root, if any was configured.

Be sure to check the surrounding API docs when changing something.

Enable the placeholder text on the editing root, if any was configured.

"if any was configured" part is no longer true after your changes. Because placeholder mechanism is added anyway and it listens to the placeholder property.

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Logic like that:

ckeditor5/packages/ckeditor5-editor-balloon/src/ballooneditorui.ts

Lines 108 to 128 in ce44c03

	private _initPlaceholder(): void {
	const editor = this.editor;
	const editingView = editor.editing.view;
	const editingRoot = editingView.document.getRoot()!;
	const placeholder = editor.config.get( 'placeholder' );
	if ( placeholder ) {
	const placeholderText = typeof placeholder === 'string' ? placeholder : placeholder[ editingRoot.rootName ];
	if ( placeholderText ) {
	editingRoot.placeholder = placeholderText;
	}
	}
	enablePlaceholder( {
	view: editingView,
	element: editingRoot,
	isDirectHost: false,
	keepOnFocus: true
	} );
	}

Is repeated in multiple places. It would be a good idea to make a common helper reused by all the editor UIs.

[mlewand]

I'll handle the review comments to conclude this PR.