associate messages with no parent to all qt frontends

The only messages that should have no parent are print statements made prior to the first execution.  The principal example of this is the pylab welcome message.

IPython/frontend/qt/base_frontend_mixin.py

@@ -106,4 +106,9 b' class BaseFrontendMixin(object):'
             from this frontend.
         """
         session = self._kernel_manager.session.session
-        return msg['parent_header']['session'] == session
+        parent = msg['parent_header']
+        if not parent:
+            # if the message has no parent, assume it is meant for all frontends
+            return True
+        else:
+            return parent.get('session') == session