Activating a "#foo" fragment link will now be handled internally by
the Frame instead of involving the widget layer.
If the viewport needs to be scrolled as a result, we will simply ask
the PageClient to scroll a new rect into view.
* A PageView is a view onto a Page object.
* A Page always has a main Frame (root of Frame tree.)
* Page has a PageClient. PageView is a PageClient.
The goal here is to allow building another kind of view onto
a Page while keeping the rest of LibWeb intact.
When a paint invalidation occurs inside a subframe, it bubbles up to
Frame::set_needs_display(). From there, we call PageView if this is
the main frame, or otherwise invalidate the subframe host element.
