Refactor ItemContainerGenerator API

The ItemContainerGenerator API needs a rework; in particular its materialized events are rather confused and don’t really fire when they should.

We need to decide a few things:

• Should we follow the WPF API for ItemContainerGenerator? Personally I’ve always found this API confusing as it splits container generation between the ItemsControl and the generator, with the generator just being used to signal that it’s creating “batches” and the actual containers being created by the ItemsControl itself with a bunch of overridable methods there
• The UWP API seems to be slightly simpler but I’m unfamiliar with it
• The WinUI ElementFactory API is much nicer though it still has things I don’t like much - mainly the fact that one needs to create a new ElementFactoryGetArgs object for each container generated. In particular though the ItemsRepeater events for prepared/cleared/index changed are much nicer than having to override methods in WPF/UWP

We probably want a hybrid of these APIs, which leave the main part of the API unchanged from 0.10.x for the simple use-case of defining a new container type.

[grokys]

@Tulesha opened a PR to add support for this: https://github.com/AvaloniaUI/Avalonia/pull/11068

[grokys]

Should Items vs ItemsSource be considered again as well: https://github.com/AvaloniaUI/Avalonia/issues/7553

Yep, this will be addressed before 11.0!