BindingContext object for making it easier to navigate from things like markup extensions and value converters and other places

[einari]

Navigation of the DataContext in XAML is a pain and it really pollutes the binding expressions. It also often links you directly to the XAML structure making it fragile. While the goal is actually very simple; we want to access the parent DataContext and perhaps even navigate further. This could easily be a source markup extension that could be provided that has parameters telling how far up one wants to go. But also a programatic approach within ValueConverters and MarkupExtensions to make it possible to easily get to these.