[enhancement]: Third "target" mode

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Contact Details

_No response_

### What should this feature add?

In addition to the "to gallery" and "to canvas" modes, I propose adding a third one, "automatic". It would send the generation result to the canvas on inpainting and outpainting (i.e. all "modifying" generations), but to the gallery on txt2img and img2img (i.e. all "new image" generations).

In most cases, this is exactly the behaviour you want. Sending an inpainting layer to the gallery is pretty useless. Sending img2img to the canvas has some more, but still rare, use cases, but I'm not suggesting getting rid of the manual choice anyway.

### Alternatives

Cancelling and restarting generation every time I switch between txt2img and in/outpainting one of the results because I forgot to flick the switch, just as I do at the moment.

### Additional Content

Additionally, opening/closing the viewer based on the generation target in auto mode could be worthwhile. The "your princess is in another castle" message is nice, but switching by itself (once. when clicking "invoke") would be nicer. My estimate is that it would save a click way more often than causing one.