LibWeb: Start work towards modern CSS "display" values

Until now, we've internally thought of the CSS "display" property as a
single-value property. In practice, "display" is a much more complex
property that comes in a number of configurations.

The most interesting one is the two-part format that describes the
outside and inside behavior of a box. Switching our own internal
representation towards this model will allow for much cleaner
abstractions around layout and the various formatting contexts.

Note that we don't *parse* two-part "display" yet, this is only about
changing the internal representation of the property.

Spec: https://drafts.csswg.org/css-display

Userland/Libraries/LibWeb/HTML/HTMLBRElement.cpp

@@ -22,7 +22,7 @@ HTMLBRElement::~HTMLBRElement()
 RefPtr<Layout::Node> HTMLBRElement::create_layout_node()
 {
     auto style = document().style_computer().compute_style(*this);
-    if (style->display() == CSS::Display::None)
+    if (style->display().is_none())
         return nullptr;
     return adopt_ref(*new Layout::BreakNode(document(), *this, move(style)));
 }