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.