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/CSS/Length.cpp

@@ -53,6 +53,8 @@ const char* Length::unit_name() const
         return "rem";
     case Type::Auto:
         return "auto";
+    case Type::Percentage:
+        return "percentage";
     case Type::Undefined:
         return "undefined";
     }