- handle all sorts of image types, gegl_buffer_get() will convert to u8
- specify GIMP_EXPORT_CAN_HANDLE_LAYERS_AS_ANIMATION
- modify the export duplicate, not the original image
- fix handling of the drawable type and only save "R'G'B'A u8"
  or "R'G'B' u8", fixes bug 770664.
- completely redo run(SAVE_PROC) to deal with export correctly
- turn global variables into the standard SAVE_PROC, PLUG_IN_BINARY etc
  defines
- simplify the save dialog