New comments applied and merged.

I left one additional (trivial) comment on bitbucket.
From my perspective, this looks good to merge. :-)

I applied your suggestions

I left some comments on Bitbucket.

I started looking at this, but the pull request diff shows some merge conflicts.
Can you resolve these? I think what you need to do is merge from default and
resolve any merge conflicts, but someone with more experience with this (e.g.
Jendrik) might be able to give you better advice.

Rework ready for review:
https://bitbucket.org/PatFer/downward/pull-requests/5/issue830/diff

Rework the registry system to have one registry managing all three,
PluginRegistry, PluginTypeRegistry, PluginGroupRegistry.

One Registry to rule them all.