Fix notifications for non-visible sessions.

Currently all notifications are deemed 'visible'.  This patch fixes
that so all non-visible sessions go to their own notifications.

Patch by Tijl Coosemans.

I'll backport to the 4.7 branch.
BUG: 174505
FIXED-IN: 2.7.1
REVIEW: 101720

src/TerminalDisplay.cpp

@@ -2699,7 +2699,8 @@ void TerminalDisplay::bell(const QString& message)
     } 
     else if (_bellMode==NotifyBell) 
     {
-        KNotification::event("BellVisible", message,QPixmap(),this);
+        KNotification::event(hasFocus() ? "BellVisible" : "BellInvisible",
+                             message,QPixmap(),this);
     } 
     else if (_bellMode==VisualBell) 
     {