Add Prowl support.
Redesign Config->Notifications

- Separate class selections for NCenter, Growl and OSD
- Prowl gets it own classes, because it has a priority per class
- Join email sections
Enable notifications for errors and warnings.
Added missing prowl classes in display of Config->Notifications.
Confiig->Prowl consistent coloring.
Notifications touch-ups.
Fix error in sending OSD messages.
Create separate sections for notification types.
Re-layout Config->Notify.

Use a separate option for each class member.