session: Fix repeated launches and things that don't launch.

Prior to this commit we incorrectly did not autostart items that did *not*
provide an OnlyShowIn section. This has the unfortunate effect of preventing
items such as nm-applet, etc, from starting at login.

Secondly, we were incorrectly launching items multiple times through various
initialisation stages. Correcting this behaviour has the immediate effect that
you no longer need to move the mouse for the stage to appear, fixing whatever
odd conflict we initially had with gnome-settings-daemon and associated
helpers.