A new envrionment variable GNOME_SHELL_ENABLE_CLEANUP is added which
causes us to attempt freeing global data. The reason this isn't
enabled by default is that it's a waste of time at best, and at
worst in corner cases could cause crashes which would fill up
crash databases. Better to leave it as a developer-only tool.
Start stubbing out some cleanup in ShellGlobal.
https://bugzilla.gnome.org/show_bug.cgi?id=649517