Unity (Ubuntu) middle click title bar

To send a window to the back with a “middle click” on the title bar using Unity on Ubuntu, run the following command in a terminal:

dconf write /org/gnome/desktop/wm/preferences/action-middle-click-titlebar "'lower'"

or using the unity-tweak-tool:

-> Window Manager 
-> General 
-> Additional 
-> Titlebar Action -> Middle click 
->  from list choose :  Lower