lock:add message of sessioned on for power menu

description:
    when two users(A,B) have logged in,
    the user A run dde-lock ,and click the power menu.
    there should be a message to tell the user if shutdown or poweroff
    the computer,the programe running and some data of user B will lost!

tower:https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/410b8d2c1cc1412781bdb4431a916350/

Change-Id: If0f942ccf1d23147317d366968714f13ef0c78fe