(MODPYTHON-155) Modifications to handler and filter lists so that references can be maintained back to parent handler of a handler/filter which was registered dynamically. This is necessary within the scheme of how the new module importer works. Specifically, where a directory isn't specified when adding a handler or registering a filter, the directory associated with the parent up the chain has to be used instead. In the old importer it was relying on fact that directory of parent had already been added to the Python sys.path, although this didn't actually work reliably. The new module importer doesn't add directories to sys.path and so the need to properly work out directory associated with a parent so that it can be used when required.