LibHTML: Use an enum for CSS property ID's

Instead of using string everywhere, have the CSS parser produce enum
values, since they are a lot nicer to work with.

In the future we should generate most of this code based on a list of
supported CSS properties.

Libraries/LibHTML/CSS/PropertyID.h

@@ -0,0 +1,62 @@
+#pragma once
+
+#include <AK/Traits.h>
+
+namespace CSS {
+enum class PropertyID {
+    Invalid,
+
+    BackgroundColor,
+    BorderBottomColor,
+    BorderBottomStyle,
+    BorderBottomWidth,
+    BorderCollapse,
+    BorderLeftColor,
+    BorderLeftStyle,
+    BorderLeftWidth,
+    BorderRightColor,
+    BorderRightStyle,
+    BorderRightWidth,
+    BorderSpacing,
+    BorderTopColor,
+    BorderTopStyle,
+    BorderTopWidth,
+    Color,
+    Display,
+    FontFamily,
+    FontSize,
+    FontStyle,
+    FontVariant,
+    FontWeight,
+    Height,
+    LetterSpacing,
+    LineHeight,
+    ListStyle,
+    ListStyleImage,
+    ListStylePosition,
+    ListStyleType,
+    MarginBottom,
+    MarginLeft,
+    MarginRight,
+    MarginTop,
+    PaddingBottom,
+    PaddingLeft,
+    PaddingRight,
+    PaddingTop,
+    TextAlign,
+    TextDecoration,
+    TextIndent,
+    TextTransform,
+    Visibility,
+    WhiteSpace,
+    Width,
+    WordSpacing,
+};
+}
+
+namespace AK {
+template<>
+struct Traits<CSS::PropertyID> : public GenericTraits<CSS::PropertyID> {
+    static unsigned hash(CSS::PropertyID property_id) { return int_hash((unsigned)property_id); }
+};
+}