Description of problem:
The mouse scroll down causes pup to show the ~"no updates" text.

Version-Release number of selected component (if applicable):
pirut-1.2.8-1.fc6

How reproducible:
Yes.

Steps to Reproduce:
1. pup {Assume there is updates available, and they get shown}
2. hold mouse over the horizontal splitter
3. scroll up -> nothing happens
4. scroll down
  
Actual results:
pup body changes to the "There are no updated packages currently available for
your system" 

Expected results:
No change.

Additional info:
By making the tabs visible, I found that the mouse scroll event by default is
used to perform tab next and previous functions. However, once scroll down event
has fired the "no updates" pages is active, and there is no way to get back to
the updates list.

Adding the notebook on_scroll event, and then responding to it by doing nothing
solves the problem, by overriding the default gtk behaviour.

ps. Being a python/gtk newcomer there may be a more effective way to achieve
this, however the attached patch does get the job done.