You can now pass a file:/// URL to HtmlView and it will take care of
the loading logic for you. Each Document remembers the URL it was
loaded from, which allows us to also have reload().
This patch also adds a very simple function for resolving relative
URL's into absolute ones.
This patch implements basic support for presentational hints, which are
old-school HTML attributes that affect style.
You add support for a presentational hint attribute by overriding
Element::apply_presentational_hints(StyleProperties&) and setting all
of the corresponding CSS properties as appropriate.
To make the background color fill the entire document, not just the
bounds of the <body> element's LayoutNode, we special-case it in the
HtmlView::paint_event() code for now. I'm not entirely sure what the
nicest solution would be, but I'm sure we'll discover it eventually.
Each HtmlView now has a main_frame(), which represents the main frame
of the web page. Frame inherits from TreeNode<Frame>, which will allow
us to someday implement a Frame tree (to support the <frame> element.)
We now show a tooltip for the hovered node's enclosing HTML element's
title attribute, if one is present.
This patch also adds HTMLHeadingElement. The tags h1-h6 will now create
the right kind of objects.
