LibWeb: Remove unecessary dependence on Window from DOM and WebIDL

These classes only needed Window to get at its realm. Pass a realm
directly to construct DOM and WebIDL classes.

This change importantly removes the guarantee that a Document will
always have a non-null Window object. Only Documents created by a
BrowsingContext will have a non-null Window object. Documents created by
for example, DocumentFragment, will not have a Window (soon).

This incremental commit leaves some workarounds in place to keep other
parts of the code building.

Userland/Libraries/LibWeb/DOM/CharacterData.h

@@ -34,7 +34,7 @@ public:
     WebIDL::ExceptionOr<void> replace_data(size_t offset, size_t count, String const&);
 
 protected:
-    explicit CharacterData(Document&, NodeType, String const&);
+    CharacterData(Document&, NodeType, String const&);
 
 private:
     String m_data;