LibWeb: Add Origin concept (protocol, host, port tuple)

Every Document now has an Origin, found via Document::origin().
It's based on the URL of the document.

This will be used to implement things like the same-origin policy.

Libraries/LibWeb/Forward.h

@@ -45,6 +45,7 @@ class LayoutDocument;
 class LayoutNode;
 class MouseEvent;
 class Node;
+class Origin;
 class Selector;
 class StyleResolver;
 class StyleRule;