A lot still broken, in particular virtualization is completely removed.`ItemsPresenter` now no longer has an `Items` or `ItemTemplate` property; it detects when it's hosted in an `ItemsControl`. `IItemsPresenter` interface removed.

Currently only supports smooth scrolling.

If you want a different virtualization mode, use a different panel.

A bunch of tests still failing, and some code commented out, but outlines the new API shape.

`ListBox` now needs a root as it uses a `VirtualizingStackPanel`.

Use a slightly different approach to the one that was previously there: create an item template that contains `DisplayMemberBinding`. This is the approach that WPF broadly uses too.

When `ItemsControl.DisplayMemberBinding` or `ItemTemplate` changes, refresh the containers.

As `DisplayMemberBinding` should select the member for the header (in WPF it's `DisplayMemberPath`). Also set `TabItem.TabStripPlacement`.

Wasn't finding `RelativeSource` type at times.

Only `VirtualizingCarouselPanel` currently implemented.

Now that containers are being recycled, the state of the control following the animation needs to be the same as before the animation.

Fixes #54.

So in future we could have different `ItemSourceView` concrete implementations that e.g. have an inner `IReadOnlyList<T>` in order to address #8764.

Only invalidate the measure when the viewport changes in the direction of scroll.

Variable width was causing the panel to flicker around as the values change.

Prevents compiled bindings failing because the `DataContext` is not yet set and is still inheriting the parent `DataContext`.

Don't allow child menus to inherit the top level `Menu`'s `ItemContainerTheme` as that is specific to top-level menu items.

This necessitated adding another stage to the container preparation lifecycle that will be called after the container is added to the logical and visual trees: `ItemContainerPrepared`. Such a method is going to be necessary anyway if we ever want to expose events for container realization so will probably be worth having regardless.

And added some more doc comments.

It's a TreeDataGrid concept (which is where this code was adapted from).

Somehow got changed to an older commit.

`VirtualizingStackPanel` only does smooth scrolling now.

Trying to get a repro for an issue seen with DataBox but so far unable to repro in a unit test. Anyway, these tests should be useful.

There's a problem with `DataBox` that I can't repro in a unit test. Add some redundancy such that if we're still waiting for a viewport update after carrying out a layout pass, force another layout.

Fixes #9269