applets/notifications: Notifications refactor

Reorganize delegate loading for notifications:

• split things into logical components, each smaller sub-component is easier to maintain/debug/rewrite
• move to an architecture where different kinds of notifications are implemented by a completely different type of delegate, with less multi-behavior giant delegates
  • PopupDelegate has a PlasmoidHeader and a ScrollView
  • HistoryDelegate looks similar to the popup one but doesn't have header and scroll
  • HistoryGroupedDelegate doesn't have an header, has close button near the summary and a gray line on the left
  • Since no delegate in the history has a ScrollView anymore, WheelForwarder is not necessary anymore
• Everything concerning delegates is in the delegates subdirectory
• Encapsulation has been sacrificed a bit in favor of a central class to interact with the model, ModelInterface that has properties and signals. This avoid long binding chains and long boilerplate code repeated in many places
• Total instantiated QML items per-delegate should be significantly less
• look and feel should be completely unchanged, if any UI change is to be done, should be on a separate mr