Adds buttons to send selected output images to ControlNet #0

• Move buttons next to/directly below the default "Send to..." buttons
• Support extra ControlNet Units
• Cleanup code

Note that further development is low priority since it already meets my usecase. Hopefully the ControlNet authors/contributers will get around to a better implementation than this hacky one.

https://github.com/Navezjt/sd-webui-photopea-embed <- Image transfer related code referenced/copied.

https://github.com/AlUlkesh/sd_delete_button <- Button related code referenced/copied.