…ranch.

…erting elements (text nodes are not supported).

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.

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

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

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

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

…ent handler.

…ToElement

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

…r-event-flow

# Conflicts:
#	packages/ckeditor5-engine/src/conversion/downcasthelpers.js

…r-event-flow

…cast converters.

…event-flow

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.

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

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

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

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

Tests (clipboard): Added missing downcast conversion.

Internal (heading): Added missing consuming converter.

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

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`](https://ckeditor.com/docs/ckeditor5/latest/framework/guides/support/error-codes.html#conversion-model-consumable-not-consumed) error documentation for more information.

…shItem().