util: Add session env variables to child env
GNOME Session does not replicate all the environment variables to its children while it sets them as activation environment for both DBUs and Systemd.
This is causing the g-s child not to use pre-set variables such as XDG_RUNTIME_DIR for launching its children such as the whale dialog.
To fix issue #15 actually would be just needed to call
XDG_RUNTIME_DIR but since these variables are already passed to both dbus and systemd I expect also gnome-session to use them fully.