WIP: Try/navigator in sidebar for nav and menu items

[talldan]

Description

Experiment to test the Block Navigator in the sidebars of both the Navigation Menu and Menu Item blocks.

This has been done in a very hacky way, it's just a proof of concept right now, not meant to be code reviewed.

This is an iteration from #18202

Screenshots

[talldan]

I think this works really nicely. It has the caveat that it only works so seamlessly when it's rendered in the same position for the parent and child blocks. If the descriptions of the two blocks were different lengths there'd be a noticeable jump of the position of the panel.

In terms of implementation, at the moment I'm rendering the same component in both the Nav Menu and Menu Item InsepectorControls. There's a lot of coupling between the two blocks in the way this is done, which is less than ideal.

I wonder if a better approach would be to create a slot that's like InspectorControls, but perhaps called RootInspectorControls, and it renders both when the parent block is selected, but also when any inner block is selected. For example the nav menu would just have this:

<RootInspectorControls>
	<PanelBody
		title={ __( 'Navigation Structure' ) }
	>
		<BlockNavigationList clientId={ clientId } />
	</PanelBody>
</RootInspectorControls>

and rendering would be taken care of for all inner menu items.

[mtias]

This looks very promising and might be an indication we should turn the top level block navigator into a sidebar so that it can remain open if you want to dig into structure.

[talldan]

I refactored this along the lines mentioned in comment #18203 (comment). The Navigation Structure panel is now only rendered in the navigation menu in a ParentInspectorControls slot, but this is slot still renders when a child block is selected. It works pretty well.

When experimenting with larger navigation menus, the panel does get quite big, so I think I'll next try out a sidebar tab as @mtias mentions #18203 (comment).

[karmatosed]

Noting #18340 has some points relating to this. Specifically, we now have 2 places.