unlockDialog: Fix scale-factor handling on multihead

The blur effect needs to take the scale-factor into account, so we listen for scale changes. However we set up the signal handler when creating a background, which is repeated for each monitor, and every time the monitor configuration changes. But we only disconnect the last handler that was connected, and only when we are destroyed, not when recreating backgrounds.

Fix this by splitting out updating the effect parameters to a separate method that iterates over all backgrounds, so we can simply set up the handler from the constructor.