[extensions] Add OutgoingFileTransfer.setCallback()

Some methods of OutgoingFileTransfer take already the
NegotiationProgress callback as argument, some do not. The API is not
ideal, and adding a setter for the callback at least makes it possible
to always have the callback.

smack-extensions/src/main/java/org/jivesoftware/smackx/filetransfer/OutgoingFileTransfer.java

@@ -324,6 +324,10 @@ public class OutgoingFileTransfer extends FileTransfer {
         transferThread.start();
     }
 
+    public void setCallback(NegotiationProgress negotiationProcess) {
+        this.callback = negotiationProcess;
+    }
+
     private void handleXMPPException(XMPPErrorException e) {
         StanzaError error = e.getStanzaError();
         if (error != null) {