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

js/ui/runDialog.js

@@ -189,7 +189,7 @@ __proto__: ModalDialog.ModalDialog.prototype,
                                    }),
 
                                    'debugexit': Lang.bind(this, function() {
-                                       Meta.exit(Meta.ExitCode.ERROR);
+                                       Meta.quit(Meta.ExitCode.ERROR);
                                    }),
 
                                    // rt is short for "reload theme"