Add support for checkpoint merging

adds support for a notification.mp3 in the root directory that will play upon completion (fixes #1013)

ddim_eta & ddim_discretize

refactored image paste handling to fill unset images successively, then replace last existing image (fixes #981)

Fixed grids saving to subdirectory when the setting is unchecked but "save images to subdirectory" was checked.

Fix by @MrAcademy

adds support for a notification.mp3 in the root directory that will play upon completion (fixes #1013)

copy over fields when sending to img2img: https://github.com/AUTOMATIC1111/stable-diffusion-webui/issues/1029