Fixed hopefully the ScrollArea segfault. Fixes #45

The Gui now sets the VisibilityEventHandler to Null when it is destructed. That should prevent segfaults that are occured by Widgets that sends Events after Fifechan is destructed.

src/gui.cpp

@@ -131,6 +131,7 @@ namespace fcn
         {
             setTop(NULL);
         }
+        Widget::_setVisibilityEventHandler(NULL);
         Widget::_setGuiDeathListener(NULL);
 
         delete mFocusHandler;

src/widgets/scrollarea.cpp

@@ -145,9 +145,7 @@ namespace fcn
 
     ScrollArea::~ScrollArea()
     {
-		// Is this really needed? Commented out for the moment, because it tends to segmentation faults.
-		// See widget.cpp clear()
-        //setContent(NULL);
+        setContent(NULL);
     }
 
     void ScrollArea::setContent(Widget* widget)