app, pdb, libgimp: allow to choose the channel when thresholding
so the threshold can now be based on the GimpHistogramChannel enum. Add a channel menu to the threshold dialog and a channel argument to the PDB procedure (which is new in 2.10). If I hadn't forgotten what the "RGB" channel is supposed to do I would have implemented the RGB mode in GimpOperationThreshold correctly. Right now I'm just guessing. Anyone?
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