Global: Make SeekOrigin an enum class

Makes the enumeration strongly typed and also allows forward declaring
the enumeration type as well.

include/athena/Global.hpp

@@ -123,7 +123,7 @@ namespace athena {
 namespace error {
 enum class Level { Message, Warning, Error, Fatal };
 }
-enum SeekOrigin { Begin, Current, End };
+enum class SeekOrigin { Begin, Current, End };
 
 enum Endian { Little, Big };