Add manual lens correction module
I'd like to suggest a new module: manual lens correction. I imagine this would have a single slider for barrel/pincushion correction equivalent to lensfun's "b" parameter. (Sliders for "a" and "c" could be a bonus though I believe any corrections beyond the most obvious are hard to do based on visual feedback.)
Alternatively, the existing lens correction module could be extended to have manual settings.
Rationale: Not all camera types and lenses are present in the lensfun database and it will always lag behind. An end user may create a lensfun profile for their equipment but should not be required to if they just want to correct a couple photos. This feature would be a quick-and-dirty solution, and also something many other raw workflow applications already have.