For pages that can pop back (e.g. Partial pages \ View Components), the
invoking component already creates a new ViewContext. ViewStart, Pages
and Layouts need to share the same Layout. This is required for sharing
ViewData values (such as title).
Fixes#861
* Introducing RazorPage and RazorPageOfT that represent the Razor
execution aspect of view execution. Moving view execution hierarchy behavior
(Layout, partial views etc) into a separate RazorView type.
* Renaming IVirtualPathViewFactory to IRazorPageFactory,
IRazorViewActivator to IRazorPageActivator
* Renaming VirtualPathViewFactor to FileBasedPageFactory to
correctly reflect what it does.
Fixes#814
Pranav K