[GUI] added simple notification for Whiteboard session establishment

I was just reading

 schwieriger Start ... beim Verbinden kommt "Synchronisiere Nutzerliste"
- wenn                                                               
dieser Vorgang abgebrochen wird, dann funktioniert alles fehlerfrei

Well, there is no chance to abort the user list synchronization ... what
the user aborted was the Whiteboard synchronization. Such feedback is
not very useful for new Saros Developers as this is somewhat misleading.


Change-Id: I09c3a73144eaea1a06fd1c980a9f3690194a456f
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1354
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>