Update the XML module to work with the modified API.

Separate the XML handles into their own source files.

modules/xml/Handle/Document.cpp

@@ -0,0 +1,16 @@
+// ------------------------------------------------------------------------------------------------
+#include "Handle/Document.hpp"
+
+// ------------------------------------------------------------------------------------------------
+namespace SqMod {
+
+// ------------------------------------------------------------------------------------------------
+void DocumentRef::Validate() const
+{
+    if (!m_Ptr)
+    {
+        STHROWF("Invalid XML document reference");
+    }
+}
+
+} // Namespace:: SqMod