kcm/notifications: Add "System Notifications" button
Add "System Notifications" button
It’s an implementation detail that generic notification sounds for message boxes are in a "Plasma Workspace" category.
Instead, link them from the KCM main page.
Replace EventsProxyModel with DelegateModel
Simplifies the code and the custom proxy did nothing more than show a specific branch of the tree model, something DelegateModel can also do.
Superseds !2001
