app: fix wrong extension active state on exit.
First the deserialize data of extensionrc was wrong. I need to edit the pointer to the GList (and dereference it when I need). Also when inserting an extension into the `running_extensions` hash table, I could not reuse the same string as in the `processed` list (because this string is going to be freed at end of initialization). Just always use the GimpObject name of the extension, since it will stay alive for as long as the object is alive.