* NEWS item
* added `onStop` arg to `shinyApp()` (and renamed our internal `onEnd` - which is what was calling `on.exit()` already - to `onStop` as well)
* added onStop() function
* add entry for documentation
* make it work for all possible app structures (interactive, saved as app.R, saved as ui.R and server.R)
* fix#1772: make sure `onStart` works in all scenarios
* update NEWS
* improved wording
* more wording
* and more wording
* don't stop execution if a `onStop` callback function results in an error
* remove "(all sessions have been disconnected)" because it's misleading
* add @seealso documentation
* shamefully forgot to Cmd Shift D
* change code place
* Code review feedback
* onStop: use session argument instead of scope
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