Move notifications constants, add convenience
Decouple syncPushNotificationInfo

Registers and gets settings via XML RPC API
Decouple saveNotificationSettings

- Move app usage into NotificationsManager

- Make WordPressComApi app-independent
Decouple fetchNotificationSettings

- Move app usage into NotificationsManager

- Make WordPressComApi app-independent
Decouple notifications unregistration

- Move app usage into NotificationsManager

- Make WordPressComApi app-independent
Remove notification from WordPressComApi

- Notification was posted for checkForUnseenNotifications

- Also turns out that this method is called, but nothing observes the notification

- Removed the call from the app in NotificationsManager.