Curve widget layout support

Some of Gegl's image-processing filters rely on curves, those draggable graphs you can bend to fine-tune how an effect behaves. Up until now, only a couple of built-in tools (in GIMP) had a proper way to show and edit those curves. If a filter needed one, there simply wasn't a ready-made widget for it.

A merge request currently in progress https://gitlab.gnome.org/GNOME/gimp/-/merge_requests/2901 aims to fix that gap by adding generic curve editing support to GIMP's filter dialogs. In practice, this means any filter that uses a geglcurv type property will automatically get an interactive graph, complete with draggable points and precise numeric input.

Work on this has moved in stages: first making sure curve settings could be properly saved and duplicated, then wiring up an actual visual widget, and now refining it to use GIMP's own richer internal curve format so nothing gets lost along the way. A few tricky bugs have popped up, like curve edits not immediately taking effect when reopening a filter.

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