Signals the client that the session manager will soon want the application to quit, perhaps as a part of ending the session (but this should not be assumed).
The application can take this opportunity to prompt the user to save any unsaved work to disk.
This signal also expects a return value from the handler. If the application wishes to cancel the quit request (perhaps because the user selected "Cancel" in prompts to save unsaved work), it should return true from the handler. If the application is satisfied with possibly needing to quit soon, the handler should return false.