LibWeb: Move width into LayoutStyle

This patch also adds the ability for Length to contain percentage
values. This is a little off-spec, but will make storing and dealing
with lengths a lot easier.

To resolve a Length to a px-or-auto Length, there are now helpers
for that. After calling them, you no longer have to think about
em, rem, %, and such things.

Libraries/LibWeb/Layout/LayoutNode.cpp

@@ -226,6 +226,7 @@ void LayoutNodeWithStyle::apply_style(const StyleProperties& specified_style)
     style.set_position(specified_style.position());
     style.set_text_align(specified_style.text_align());
     style.set_z_index(specified_style.z_index());
+    style.set_width(specified_style.length_or_fallback(CSS::PropertyID::Width, {}));
 }
 
 }