kit: mark Document as final

Document has to be final: its constructor calls sendTextFrame(), which
calls the virtual sendFrame(). So the only situation where this code
works correctly is when Document is never subclassed.

Change-Id: I7c13abdae57d9e1ba3416ec1963b58bce0d42e6f

kit/Kit.cpp

@@ -855,7 +855,7 @@ public:
 /// per process. But for security reasons don't.
 /// However, we could have a loolkit instance
 /// per user or group of users (a trusted circle).
-class Document : public DocumentManagerInterface
+class Document final : public DocumentManagerInterface
 {
 public:
     /// We have two types of password protected documents