lxde-metapackages has two dependency error

[elwin]

on ubuntu 16.04, apt-get install lxde，then reboot the system, can't
find lxde menu item in the lightdm display manager,

we should apt-get intall lxsession
then reboot the system, then can find the lxde menu item in the lightdm display manager.

after login the lxde desktop envirionment ,we can't logout the session by click the logout menu item
in the startmenu, we should apt-get install lxsession-logout ,install the /usr/bin/lxsession-logout
program, then we can logout the lxde by clicking the logout start menu item in the start menu.

we find this bug early in the early 2016 October but reported it so late.

for more detail infomation can visit this URL.
https://elwin-mao.github.io/ubuntu_16.04_lxde_error.html

moreover: ubuntu14.04 and ubuntu17.10 have no this problem. and the ubuntu 16.04 is not reached EndOfLife now!!!!
we need fix it~