wheezy: add a quick shutdown button back

We need to add our quick shutdown applet (the red button on the far right of the top GNOME panel, in Tails 0.x) back.

That probably means adapting our applet to work with GNOME Panel 3.4.

Additional information:

The current applet lives in config/chroot_local-includes/usr/local/bin/shutdown_helper_applet.
The applet is not available in the Add to Panel menu on Wheezy.
TailsClock might be a good example as it provides an applet that runs both on GNOME2 and GNOME3.

The code should be based off the feature/wheezy branch.

A first step is probably to adapt the applet so that one can manually add it to the panel
Then, one should register the applet somehow, to have it shown by default (probably related to #6381 (closed)).

Feature Branch: bugfix/5355-add-a-quick-shutdown-button-back

Parent Task: #6015 (closed)

Related issues

Blocks #6004 (closed)

Original created by @tails on 5355 (Redmine)

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