Code review comment for lp://qastaging/~kamstrup/dee/filter-models-ng

I'm getting: `Warning: multiple "IDs" for constraint linkend: DeeFilter.` when generating the documentation, not sure what's the root cause of that, but it wasn't there in trunk.

What's missing is regenerated vapi (we really need to hook that up into the build system properly) + no longer forcing Dee.Filter to be compact class.

One of the things I was never sure about is whether GI-langs are able to use functions that have in the C signature two function pointers and a single userdata + destroy (ie current dee_filter_new), but it doesn't seem likely.