DataTemplate known issues

[Dreamescaper]

• It is possible to create DataTemplate from RenderFragment only, and it requires a BindingContext to be set.
  I plan to add support for RenderFragment (non-generic), which doesn't require BindingContext later.

• Null items are not supported at this point.
  As I mentioned, I decided not to support nulls at this point. I did that to avoid unnecessary updates (CollectionView sets BindingContext to null before changing it to other value for some reason), and because I wanted to avoid the requirement to handle nulls in RenderFragment even when all items provided are not null.

• Every element created via MbbDataTemplate has an additional ContentView parent. Sometimes that's OK, but probably not always.
  Have no idea how to avoid that.

• Can't use this approach for ShellContent.ContentTemplate.
  That's a specific case of the previous item. In this case created element should be Page element. But in our case it is wrapped in ContentView, which doesn't work here.
  Maybe it would be possible to handle this properly in different way, not sure.