Fix warnings and clean up public API headers

[drobilla]

Fixes various warnings that the public API headers can trigger in client code, and does some tidying up along the way.

Shouldn't be any non-superficial changes.

[drobilla]

@falkTX Going over my open tickets/PRs, this one seems relatively uncontroversial. It'd be nice to at least get the warning fixes in there; thoughts?

[falkTX]

yeah I need to start merging jack stuff and do releases, which for the headers needs to be done on both jack1 and jack2 side so the eventual headers repo reusage does not become a big deal. I am in the middle of moving once again, back to berlin, will take care of jack things once things are a bit settled on my side.

[drobilla]

👍 no worries/rush. Just doing some new year tidying up