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.
This patch makes StyleProperties heap-allocated and ref-counted so that
a LayoutNode can be without one. The ref-counting also allows anonymous
blocks to share style with their parent block.
LayoutText never needs a StyleProperties, since text always inherits
style from its parent element. This is handled by style_properties().
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.
This also fixes another bug with inline wrappers. Namely,
we should only add inline wrappers if a block node has
both non-block (inline or text) and block children.
