Notification: Respect NotifyOSD-preference and allow testing UI values

NotifyOSD preference was never respected. Fixed.

Testing is done on saved values, not UI values. Fixed, and made consistent with email.

I've tested everything I can on my Linux machine, but I don't have a Mac so I cannot test that growl works 100% beyond attempting to make connections to machines which will reject it. If you have a Mac handy you migth want to double-check that.

Apart from that, I think this should be good. Any objections or suggestions for improvements?