Add ability to scroll through group notifications

[tiagoporsch]

Hello,

I searched around a little but could not find the means to achieve the functionality that I was looking for, so I've added a way to scroll through group notifications using the mouse wheel.

I've tested it around a bit and it seems to work fine, but I understand that the way I've implemented it is probably not the best, so any further contribution would be much appreciated.

To-do:

• Add an option to disable, or make this functionality opt-in
• Make this work using a touchpad

Let me know what you think about this!

[tiagoporsch]

@emersion any feedback on this idea? I'm still running this branch without any problems, and I think it's very important to be able to see more than the latest/earliest notification in a group.