Application state notifications

[mouselangelo]

Implements #271

Type of change: Feature

Motivation (current vs expected behavior)

iOS provides application state change notifications via NotificationCenter, which can be useful. Also, including the NotificationCenter and related types, it becomes possible for developers to add and post their own custom notifications in their apps.

Please check if the PR fulfills these requirements

• [x] Self-review: I am confident this is the simplest and clearest way to achieve the expected behaviour
• [x] There are no dependencies on other PRs or I have linked dependencies through Zenhub
• [x] The commit messages are clean and understandable
• [x] Tests for the changes have been added (for bug fixes / features)

[mouselangelo]

@ephemer - Please check this once again. The class approach is definitely more extensible in case something needs to be "overridden" in the future, even if that doesn't seem likely sometimes.

[ephemer]

@mouselangelo I agree. LGTM merging