Add an option for running in the background function

@djbeatroot commented on [Thu Feb 04 2021](https://github.com/AdguardTeam/AdguardForMac/issues/824)

### Steps to reproduce

1. Click the close/red X icon on the AdGuard window title bar.
2. A dialog box appears with a "Keep AdGuard running in the background?" question.
3. Select the "Remember my choice" checkbox.
4. Click the "No" button.

### Expected behavior

AdGuard should follow the "Remember my choice" = "No" selection every time the window is closed.
There should also be a way to view and change that selection in the settings window.
### Actual behavior

AdGuard honors the "Remember my choice" = "No" selection only the first time the window is closed. In subsequent sessions, AdGuard stays running after the window is closed. There is no visible way to view or change this selection.

Screenshot:

### Customer ID

### Your environment

* Environment name and version: (e.g. Chrome 59):
* Any specific potentially conflicting software installed: (e.g. antiviruses, firewalls, traffic counters, cleaners)
macOS Big Sur 11.2
Safari 14.0.3