WIP: stop tracking when destroy signal is received

[xai]

When the destroy signal is received and the gnome extension is disabled, e.g., by locking the screen, it is safe to assume that the current activity can be stopped.

Fixes #354

How to test:

1. Start tracking an activity
2. Lock the gnome-session
3. Unlock the gnome-session
4. Verify that no activity is being tracked

[xai]

Needs to be revised, as noted in https://github.com/projecthamster/hamster-shell-extension/issues/354#issuecomment-1412782476.

[mwilck]

As noted above, this feature doesn't belong in the extension. It should be implemented in hamster itself.