Introduce an elementToStructure and rangeToStructure downcast conversions

[niegowski]

📝 Provide a description of the new feature

(names of the downcast helpers are yet to be chosen)

Currently, while down-casting model structures to the view structures we rely on the model-view bindings (mappings) so the children of some model element are inserted as children of the bound view element. But sometimes we need to be able to indicate where some model element children should be placed after conversion.

For example for tables:

<table headingRows="1"></table>

This could be handled by the conversion as specifying a template:

<figure class="table">
	<table>
		<thead>
			<$slot for-children={ child.name == 'tableRow' && child.index < getAttribute( 'headingRows' ) }/>
		</thead>
		<tbody>
			<$slot for-children={ child.name == 'tableRow' && child.index >= getAttribute( 'headingRows' ) }/>
		</tbody>
	</table>
</figure>

The tableRows would be converted by dedicated elementToElement converters. 

This way we could use triggerBy API for headingRows attribute to reconvert only the table-thead-tbody structure without reconverting the whole table.

This would be also helpful for document/advanced lists. For example for such model structure:

<paragraph listItem="a" listIndent="0">A</paragraph>
<paragraph listItem="a" listIndent="0">B</paragraph>
<paragraph listItem="b" listIndent="0">C</paragraph>
<paragraph listItem="c" listIndent="1">D</paragraph>
<paragraph listItem="c" listIndent="1">E</paragraph>
<paragraph listItem="d" listIndent="1">F</paragraph>
<paragraph listItem="b" listIndent="0">G</paragraph>

We could generate:

<ul>
	<li>
		<$slot for-element={ paragraph with A }/>
		<$slot for-element={ paragraph with B }/>
	</li>
	<li>
		<$slot for-element={ paragraph with C }/>
		<ul>
			<li>
				<$slot for-element={ paragraph with D }/>
				<$slot for-element={ paragraph with E }/>
			</li>
			<li>
				<$slot for-element={ paragraph with F }/>
			</li>
		</ul>
		<$slot for-element={ paragraph with G }/>
	</li>
</ul>

So for the triggerBy range API we could reconvert only the ul-li structure and reuse the view of the content (paragraphs or other blocks inside those list items)

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If you'd like to see this feature implemented, add a 👍 reaction to this post.

[niegowski]

Work on this issue will be handled here: #10054