AppKit: add NSPopover notifications

Consolidate Notification EventArgs augmenting classes into
EventArgs.cs (similar in spirit to Enums.cs) since this is
going to be a common pattern.
AppKit: more notifications

- NSScroller:
  - NSPreferredScrollerStyleDidChangeNotification
- NSView:
  - NSViewFrameDidChangeNotification
  - NSViewFocusDidChangeNotification
  - NSViewBoundsDidChangeNotification
  - NSViewGlobalFrameDidChangeNotification
  - NSViewDidUpdateTrackingAreasNotification