Improve image previews in subgraphs

### Is there an existing issue for this?

- [x] I have searched the existing issues and checked the recent builds/commits

### What problem are you experiencing?

The new widget promoting method supports image previews, which is great !

But in some cases this leads to duplicate images.

### When does this problem occur?

When we have a `KSampler` and a `Preview Image` node in the subgraph, and both previews are "promoted".

### How often do you encounter this problem?

Multiple times per day

### How much does this problem affect your workflow?

Causes moderate inconvenience

### Current workarounds

_No response_

### Ideas for solutions (Optional)

When we have a `KSampler` and a `Preview Image` node in the subgraph:

- disable the original KSampler "live" preview ( to avoid duplicated images )
- show only the KSampler "live" preview, while the image is being generated.
- show only the final image, when the generation is done.

---

Now we see 3 images at once, but I only need to see 1


### Additional context

I think this was also the way it worked when we had group nodes.

We did not have duplicated images, when we had a group node with: `KSampler` + `VAE Decode` + `Preview Image`.
The "live" preview was replaced with the final image, once the generation was done.

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-5931-Improve-image-previews-in-subgraphs-2836d73d36508167a56df03da0a80251) by [Unito](https://www.unito.io)