Add Discard button. (Testers needed!)

[timntorres]

Fulfills #3766. My first attempt both at writing a feature and changing the UI. Thorough reviews welcome and testing needed. Especially if your workflow involves high step counts or Generate forever, please see if you find this useful.

Original specification met: - Add a "Discard" button that skips AND deletes in-progress images.

Original specification avoided: - Disable Discard when Batch Size is greater than 1.

Tested and works with... ✅img2img and txt2img, ✅Batch size: 1 and Batch size: 2, ✅Batch count: 1 and Batch count: 10, ✅Generate forever and regular Generate, ✅Windows Firefox; Windows Chrome; iOS Safari ✅Show Progress enabled and disabled

Lastly, I know we're pressed for space in the UI; I mostly tossed this together just to see if I could. Enjoy!

[zer0TF]

Testing notes:

txt2img

✅ Tested Generate Forever: "Interrupt" and "Skip" still worked as intended, "Discard" did not write any files and continued processing. ✅ Tested Batch Count = 4: Verified "Skip" wrote and skipped the image, "Discard" discarded the image, and "Interrupt" wrote the final file and terminated the process. ✅ Tested Batch Size = 2: Verified all buttons worked accordingly ("Discard" discards all images in the batch, other button functionality remains the same).

img2img

✅ Tested Generate Forever: Same behavior as above (all functionality appears correct).

Questions

Unsure about the UI - how else would we have three main choices for the buttons at the top like this? Does "Interrupt" and "Skip" even make sense when "Generate Forever" is on? These questions may be outside the scope of this PR, but it's worth noting.

[zer0TF]

@timntorres May just want to mark this as "Ready for Review" - dunno how many other testers you're going to get. Best to let A1111 figure out the best way to handle the UI.

[timntorres]

I've closed this due to lack of interest.