Add attribute enhancing AllowedTypes on ContentArea/ContentReference usages

[tomaszmadeyski]

Currently, we can make use of [AllowedTypes] attribute to narrow down the selection of items that can be used in ContentArea or ContentReference fields

[AllowedTypes(AllowedTypes = new[] { typeof(ArticlePageBase) })]
public virtual ContentArea RelatedArticles { get; set; }

[AllowedTypes(AllowedTypes = new[] {typeof(Image)})]
public virtual ContentReference Flag { get; set; }

It would be nice to have a tool that allows us to narrow it down in a smarter way, by introducing custom logic.

For example: I want to be able to allow only items that are located under certain node in the tree

This tool (an attribute?) should behave in the same way as [AllowedTypes] attribute, meaning that:

• I can't drag&drop items that are not valid
• I can't choose items from content tree that are not valid
• I see a description of restrictions in the ContentArea UI control ("You can add these content types")