Add shell_global_gc

Primarily useful for debugging.  This was originally a patch against
gjs: https://bugzilla.gnome.org/show_bug.cgi?id=595323

But rather than add a random toplevel module just for this function
effectively, we can easily enough define it here.  Adding toplevel
modules to GScript should have a bit more formal process to it.

https://bugzilla.gnome.org/show_bug.cgi?id=608095

src/shell-global.h

@@ -74,6 +74,8 @@ gboolean shell_global_display_is_grabbed (ShellGlobal *global);
 
 void shell_global_reexec_self (ShellGlobal *global);
 
+void shell_global_gc (ShellGlobal *global);
+
 void shell_global_format_time_relative_pretty (ShellGlobal *global, guint delta, char **text, guint *update_time);
 
 ClutterActor *shell_global_create_root_pixmap_actor (ShellGlobal *global);