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.
