This should enable to destinguish between IFrame, Reload and Navigation
motivated loads in order to call the appropriate hooks.
This change is motivated as loading the IFrame test page causes the
IFrame url to be added to the history and shows up as the current
browser location bar.
* 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.
