This is not the exact solution to your problem, but rather a workaround. As an added bonus, using this workaround, you can shut down in one click instead of three.
Setting up a Shutdown shortcut on your desktop is easy, but first you
must change the security of the executable, because in a fresh
install, it requires the root password to be used. Open a Terminal and
enter the following command:
sudo chmod u+s /sbin/shutdown
You will be prompted for your root password. After that, you're done
and can type "exit" to exit out of the Terminal.
Next, on your Desktop, right-click on empty space somewhere and choose
to Create a Launcher. The type will be Application, which is the
default, and for the command, enter the following:
shutdown -Ph now
You may name it whatever you please, although my choice is one word, a
simple "Off", reflecting the universal term found on remote controls
and other appliances since time began. Under comments, you may wish to
note "Power down" or "Shut down" if you wish. To change the icon,
click on the springboard icon (which is the default), because we can
do better than that. I found an appropriate icon to suggest "Off"
located right here in Linux Mint 17:
After you save this, try it out and see whether it works. Mine worked
right away without a hitch. For a challenge, you might try setting up
a launcher for Restart as well. You can find all the options available
for the shutdown command by opening a terminal and typing shutdown