applets/clipboard: add privacy mode

Reason for the change

When it's enabled, any new clips will not be saved to the history.

Test plan

1. Enable privacy mode in the widget
2. Copy some text
3. The copied text does not appear in the history
4. Disable privacy mode
5. Copy some text
6. The copied text appears in the history

Screenshots or screen recordings

Bugs fixed

FEATURE: 427724 FIXED-IN: 6.2