shell: deduplicate shell signal logic
Anytime the shell sends us a signal about the end session dialog we need to do the same things: 1) stop listening for new inhibitor updates 2) stop any queued operations for redisplaying the inhibitor dialog 3) emit the signal to on the gsm-shell object so other parts of the code can listen for. We currently have a separate signal handler for every signal, and they all do they same thing. Well, they're supposed to anyway, some bits are erroneously missing from some of the handlers. This commit consolidates all the code, so we don't have it reimplemently badly in several places.
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