flatpak: Add filemonitor for keyring files

Felix Häckerrequested to merge
file-monitor into main

Closes https://gitlab.gnome.org/sophie-h/key-rack/-/issues/1

Installing entirely new Flatpaks is currently a bit tricky to test - at least on my machine because of a different issue: https://gitlab.gnome.org/sophie-h/key-rack/-/issues/14

Otherwise, the UI will update automatically as items are added/deleted as intended.

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