Greeter applies setting when user clicks "Cancel" or "Back"

Originally created by @segfault on #17087 (Redmine)

To reproduce:

1. In the Greeter, open the Additional Settings dialog (by clicking the “+” button)
2. Choose any additional setting
3. Choose one of the rows to change the setting
4. Click “Cancel” or “Back” instead of “Add”

Result:
The setting is applied, even though the user action was “Cancel” or “Back”.

In the case of MAC Address Spoofing and Network Connection, the setting is applied but not added to the list of “Additional Settings” in the Greeter window. In the case of the Administration Password, the setting is added to this list.

Feature Branch: https://salsa.debian.org/tails-team/tails/merge_requests/39/commits

Related issues

• Related to #14544 (closed)
• Blocked by #16095 (closed)