Add events to detect when the app is deactivated/activated

Add a couple of events to detect when the user deactivate the app (focus other app). In this way we can stop some timers (e.g. a blinking text cursor).

No files, tests, or entry points are named. Start by locating the library's application focus or activation handling and compare behavior across supported desktop platforms; done means exposing activation and deactivation events that can be used to stop timers such as a blinking text cursor.

Written by the indexing model from the issue text.