Update doc wrt. Tails in GNOME Boxes in Stretch

The interface of GNOME Boxes changed a bit. The shared clipboard is now deactivated by:

clicking on the hamburger menu in the top-right corner of the window
selecting “Properties” from the drop-down menu
toggling the sliding switch “Share Clipboard” from “On” to “Off” in the “General” settings tab in the opened window
closing the settings by clicking on the “x” icon in the top right of the window

Affects doc/advanced_topics/virtualization/boxes.mdwn.

The file advanced_topics/virtualization/preferences-system-symbolic.png can be removed while doing this (the menu button changed to a hamburger menu, the file is not used anywhere else).

Feature Branch: spriver:doc/12378-clipboard-in-boxes-stretch

Parent Task: #11916 (closed)

Related issues

Blocks #12432 (closed)

Original created by @spriver on 12378 (Redmine)

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