sound: Update sounds directory mtime when changing alert sound

The libcanberra event-sound-cache that stores the resolved paths for sound names can get stuck with the fallback sound (bell) after trying to play an invalid symlink, such as can happen after updating to 43 which removed some sounds which might have been selected as alert sounds before.

Neither changing the symlink nor touching the theme directory seems to invalidate that cache, but touching the sounds directory does.

This now updates the sounds directory mtime before triggering purging the sound sample cache in the sound server via g-s-d so libcanberra will load the correct sounds when repopulating the sample cache afterwards.

Closes: #2277 (closed)

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