FrameLoader now begins by constructing a DOM::Document, and then builds
a document tree inside it based on the MIME type. For text/html we pass
control to the HTMLDocumentParser as before.
This gives us access to things like window.alert() during parsing.
Fixes#3973.
LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.
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.
