[enhancement]: Add Start/End slider for Regional Guidance.

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Contact Details

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### What should this feature add?

Feature request to implement a limiting Start/End slider (similar to one in Controlnet controls) for Regional Guidance.
This feature could introduce injection of different prompts at different time during denoising process. At same time the regional guidance makes it possible to specify (relatively) the area where effect is applied.

### Alternatives

_No response_

### Additional Content

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