linux: GlobalMenu: only if UBUNTU_MENUPROXY is set

Hello,

When checking if we should react differently when GlobalMenu bar is used, we should check if `UBUNTU_MENUPROXY` env var is (correctly) set instead of checking if we're using Unity on a Unity session.

It's maybe better to do that because we can use Compiz on a Unity session without Unity. Or we can also use Unity in a different session.

Note that I was not able to test this new code (sorry for that) but when I'm using Atom on Ubuntu with a Cairo-Dock session and Unity is not running, Atom window doesn't have any menu.

A few details about my system:

    $ echo $DESKTOP_SESSION    
    cairo-dock
    $ ps aux | grep "[l]ibappmenu"
    $ echo $UBUNTU_MENUPROXY   
    
    $ echo $XDG_CURRENT_DESKTOP
    Unity

![Atom on a Cairo-Dock session](http://ibin.co/1dVcy2QOY4Vi)

On an Ubuntu session, we have something like that:

    $ echo $DESKTOP_SESSION    
    ubuntu
    $ ps aux | grep "[l]ibappmenu"
    (...)
    $ echo $UBUNTU_MENUPROXY   
    libappmenu.so
    $ echo $XDG_CURRENT_DESKTOP
    Unity

Regards,

Matt