at-spi-bus-launcher: session management fixes
at-spi-bus-launcher: session management fixes

At-spi-bus-launcher was attempting to register with gnome-session but
typically failed because it was started before gnome-session is initialized.
Now we check whether gnome-session is running and only attempt to register
if it is; otherwise watch for SessionRunning and register when se wee it.

Also, handle SessionOver.

GNOME/at-spi2-core,GNOME/at-spi2-core,GNOME/at-spi2-core