LibWeb: Make CSSStyleDeclaration an abstract class

This patch moves the CSS property+value storage down to a new subclass
of CSSStyleDeclaration called PropertyOwningCSSStyleDeclaration.

The JavaScript wrapper for CSSStyleDeclaration now calls virtual
functions on the C++ object.

This is preparation for supporting computed style CSSStyleDeclaration
objects which won't have internal property storage, but rather an
internal element pointer. :^)

Userland/Libraries/LibWeb/Forward.h

@@ -22,6 +22,7 @@ class CSSStyleRule;
 class CSSStyleSheet;
 class ElementInlineCSSStyleDeclaration;
 class Length;
+class PropertyOwningCSSStyleDeclaration;
 class Screen;
 class Selector;
 class StyleProperties;