- Load Default and Save Default for APNG options in save dialog
- More APNG options in save dialog
- DISPOSE_OP_PREVIOUS (same as GIF disposal method 3)
- BLEND_OP_* (like as GIF transparent flag, but including alpha channel)
- Fractional delay time like as 1001/30000 (delay_num/delay_den)
- Write help and i18n
