daemon: Rename GdmWelcomeSession to GdmLaunchEnvironment

"GdmWelcomeSession" was always a sort of bad name, as it is not a
GdmSession, itself (it has-a GdmSession), and it's unnecessarily generic;
it doesn't really have anything to do with "Welcome" or "Session"
itself. It managed a non-user GdmSession, spawned the process in the
correct environment (spawning a DBus daemon if need be) and made sure to
keep track of it until it died. I think "GdmLaunchEnvironment" is an
appropriate name for this.

https://bugzilla.gnome.org/show_bug.cgi?id=678057