Add a warning popup when trying to merge layers with unsaved images

[passivestar]

Trying to merge and then undo with Ctrl+Z results in a loss of data. Merge should probably check if we're trying to merge layers that have unsaved images and show a warning

screen4.mp4

[ucupumar]

Merging layers changes pixel values without painting, which most of the time is always destructive in blender afaik. I'm not sure if it is possible to fix

[passivestar]

Undo works if the images are saved though (if the blend file is saved). So it could just check if the images are saved and show a warning that it can't be undone if they aren't

[ucupumar]

Okay, I'll change the title of the issue then