GUI: Use Breeze's red icon from the Plasma panel when offline

The icon is copied from breeze/status/panel/network-disconnect.svg
because it's an icon which is a private thing intended to be used by the
Plasma shell. I do not think that I can persuade the QIcon::fromTheme to
use a particular icon from a particular directory.

Change-Id: I1b3cf14a068d8aef2ff259fa0439e87d224da0a1
CCBUG: 353369