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.c

@@ -812,7 +812,22 @@ shell_global_reexec_self (ShellGlobal *global)
   g_ptr_array_free (arr, TRUE);
 }
 
-void 
+/**
+ * shell_global_gc:
+ * @global: A #ShellGlobal
+ *
+ * Start a garbage collection process.  For more information, see
+ * https://developer.mozilla.org/En/JS_GC
+ */
+void
+shell_global_gc (ShellGlobal *global)
+{
+  JSContext *context = gjs_context_get_context (global->js_context);
+
+  JS_GC (context);
+}
+
+void
 shell_global_grab_dbus_service (ShellGlobal *global)
 {
   GError *error = NULL;