app: allow specifying a callback function for propgui pickers
Allow propgui constructors to specify an (optional) callback function when creating pickers, to be called when a color/coordinate is picked, similarly to controller callbacks. Implement picker callback support in GimpFilterTool. When the active picker has an associated callback function, call it instead of the class's color_picked() function. Add lots of "#include <gegl.h>" to .c files that miss it, which is now necessary, since this commit adds a Babl* parameter in propgui-types.h.
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