Remove `WidgetRet::Quit` and `TUIRet::Quit`: #882

Job	Run time
Check formatting	10s
[Linux, default] Build and test	45s
[Linux, native TLS + notifications] Build and test	1m 4s
Make statically linked executable (default)	1m 0s
[Linux] Make statically linked executable (desktop notifications)	2m 23s
[macOS, default] Build and test	1m 28s
[macOS, native TLS + notifications] Build and test	1m 39s
	8m 29s

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