Implement TextSearch.TextBinding

[MrJul]

What does the pull request do?

This PR implements TextSearch.TextBinding.
This is the equivalent of WPF's TextSearch.TextPath.

This new property is used in SelectedItemsControl to search for items.
Additionally, ItemsControl.DisplayMemberBinding is used as a fallback, matching WPF.

For each item, the first non-empty text from the following list will be considered:

• TextSearch.Text (if the item is a control)
• TextSearch.TextBindingProperty - New
• ItemsControl.DisplayMemberBinding - New
• Content.ToString (if the item is a IContentControl)
• ToString

Unit tests have been added.

Notes

• Needs Handle custom attributes from attached property setter kekekeks/XamlX#126 for the binding to be settable in XAML.
• This specific API shape has been requested in IsEditable combox box #18094 (comment)
• ItemsControl.BindingHelper has been extracted to its own BindingEvaluator<T> class.