Change 'debugexit' to quit main loop rather than exit(), add cleanup

Change 'debugexit' to quit main loop rather than exit(), add cleanup

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