New reconversion system

[dawidurbanski]

Suggested merge commit message (convention)

Feature (engine): The DowncastWriter#createContainerElement() should accept a list of children so bigger view structures can be created in one call. Closes #10714.

Feature (engine): The elementToElement downcast helper will log a console warning if multiple elements have been created. Closes #10610.

Feature (engine): The DowncastDispatcher will throw an error if any of the model items were not consumed while converting. Closes #10377.

Feature (engine): Introducing convertItem(), convertChildren() and convertAttributes() in the downcast conversion API interface.

Feature (engine): Added support for reconversion in DowncastHelpers#elementToElement() downcast helper. Closes #10359.

Feature (engine): Added DowncastHelpers#elementToStructure() downcast helper with reconversion support. Closes #10358.

Feature (engine): It's now possible to trigger a nested conversion while downcasting an element.

Docs (engine): Overhauled the conversion documentation and introduced the documentation for conversion helpers. Closes #10294.

Internal (engine): elementToStructure() should throw when invoked for an element that allows $text. Closes #11163.

Internal (engine): Replaced conversionApi.slotFor() in elementToStructure with writer.createSlot(). Closes #11179.

Internal (engine): Reconversion helpers can coexist with Differ#refreshItem() children.

Other (engine): Implemented the EditingController#reconvertMarker() method to be used instead of Writer#updateMarker() for marker reconversion purposes. Implemented the EditingController#reconvertItem() method to replace Differ#refreshItem(). Closes #10659.

Other (engine): The attribute and child nodes conversion events are fired by the lowest priority handler for the insert event instead of by DowncastDispatcher itself. Closes #10376.

Other (engine): Events are fired by the DowncastDispatcher even if they were previously consumed. It's the conversion handler's responsibility to check if it can be consumed or if it was already consumed by other converters.

Other (engine): The DowncastDispatcher#convert() method introduced as a replacement to previously used convertInsert(). The new method handles not only nodes conversion but also markers.

Internal (horizontal-line): The horizontalLine element downcast conversion has been changed from elementToElement to elementToStructure.

Internal (html-support): HTML elements downcast conversion have been changed from elementToElement to elementToStructure.

Internal (image): The imageBlock and imageInline elements downcast conversion have been changed from elementToElement to elementToStructure.

Internal (link): Link inlineWidget element downcast conversion has been changed from elementToElement to elementToStructure.

Internal (media-embed): The media element downcast conversion has been changed from elementToElement to elementToStructure.

Internal (page-break): The pageBreak element downcast conversion has been changed from elementToElement to elementToStructure.

Internal (widget): The blockWidget and inlineWidget elements downcast conversion have been changed from elementToElement to elementToStructure.

Internal (heading): Added missing consuming converter.

Internal (heading): Code adjusted to changes in DowncastDispatcher API.

Internal (html-embed): Code adjusted to the removal of triggerBy conversion option.

Other (list): The ckeditor5-list package was restructured into subdirectories. Closes #10811.

Other (list): Downcast conversion should consume downcasted attributes.

Other (table): Table downcast conversion migrated to elementToStructure() downcast helper. Closes #10502.

Tests (clipboard): Added missing downcast conversion.

Tests (image): Tests updated to properly handle non-consumed errors fired by the DowncastDispatcher.

Tests (alignment, autoformat, block-quote, engine, html-support, image, indent, table, word-count): Tests updated after list package restructuring.

Tests (html-support): Fixed test for checking if an attribute was consumed while converting.

Docs (ckeditor5): Links in the migration guide point to the new conversion documentation.

MAJOR BREAKING CHANGE (list): The ListEditing, ListUI, ListStyleEditing, ListStyleUI, TodoListEditing, TodoListUI plugins were moved to the dedicated subdirectories (list, liststyle, todolist).

MAJOR BREAKING CHANGE (engine): Removed the Differ#refreshItem() method from the public API. Replaced by EditingController#reconvertItem() (see #10659).

MAJOR BREAKING CHANGE (engine): The DowncastDispatcher will throw an error if any of the model items were not consumed while converting. Read the conversion-model-consumable-not-consumed error documentation for more information.

MAJOR BREAKING CHANGE (engine): The DowncastDispatcher#conversionApi property is no longer available. The instances of DowncastConversionApi being created at the start of conversion.

MAJOR BREAKING CHANGE (engine): The support for the triggerBy option for downcast helpers is removed and replaced with the new elementToStructure() options.

MINOR BREAKING CHANGE (media-embed): The createMediaFigureElement helper function first argument has been changed from writer object to conversionApi object.

MINOR BREAKING CHANGE (table): The downcast converters of the table feature has been rewritten with the use of elementToStructure() and the re-conversion mechanism. See #10502.

[Reinmar]