T/1618: Merge *cast-selection-converters into *casthelpers

[jodator]

Suggested merge commit message (convention)

Other: Merge *cast-selection-converters into *casthelpers. Closes ckeditor/ckeditor5#4455.

BREAKING CHANGE: The clearAttributes() downcast conversion helper was moved to module:engine/conversion/downcasthelpers.
BREAKING CHANGE: The convertCollapsedSelection() downcast conversion helper was moved to module:engine/conversion/downcasthelpers.
BREAKING CHANGE: The convertRangeSelection() downcast conversion helper was moved to module:engine/conversion/downcasthelpers.
BREAKING CHANGE: The convertSelectionChange() upcast conversion helper was moved to module:engine/conversion/upcasthelpers.

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Additional information

[pjasiun]

I think they should be also marked as protected exports. Note that it is fine to use the protected code in tests and related files in the same module.

[pjasiun]

Ach, I see that insertText, remove, etc. are not protected. Thet let me rethink it.

[jodator]

Ach, I see that insertText, remove, etc. are not protected. Thet let me rethink it

Yeah we might take a second look at those methods. It might be a case that those exports are needed because of dev-utils or use in controllers/editors/model but are not needed by an enddeveloper.

[coveralls]

Coverage remained the same at 100.0% when pulling 37a981f on t/1618 into 1c7ef68 on master.

[jodator]

@pjasiun mostly here:

ckeditor5-engine/src/controller/editingcontroller.js

Lines 96 to 106 in f16eeba

	// Convert selection from the view to the model when it changes in the view.
	this.listenTo( this.view.document, 'selectionChange', convertSelectionChange( this.model, this.mapper ) );
	// Attach default model converters.
	this.downcastDispatcher.on( 'insert:$text', insertText(), { priority: 'lowest' } );
	this.downcastDispatcher.on( 'remove', remove(), { priority: 'low' } );
	// Attach default model selection converters.
	this.downcastDispatcher.on( 'selection', clearAttributes(), { priority: 'low' } );
	this.downcastDispatcher.on( 'selection', convertRangeSelection(), { priority: 'low' } );
	this.downcastDispatcher.on( 'selection', convertCollapsedSelection(), { priority: 'low' } );

[pjasiun]

I was thinking about them for a while (and I even started writing a follow-up) and let's keep it the way it is. These helpers are not exposed in the editor API, you need an external import to use them. Also, I can imagine one who wants to build a tool similar to dev-utils based on them so it is fine as it is.