Update RoiTool to work with newer RectRoiGraph widget version

For context, see https://git.xfel.eu/karaboDevices/guiextensions/-/merge_requests/199 (am I using it as intended, @carinanc?)

Some observations:

Now that ROI coordinates are provided as a list after the image data path, we could support multiple ROIs. Would that be neat? I don't know what detector enthusiasts are actually looking for. Should ask one of them.

Now that the list of ROI coordinates is part of hardcoded paths near the image data node, we can decouple the image preview and the ROI. Specifically, we could point the image source to the input (from, say, a DetectorAssembler) and the ROI coordinates to the RoiTool itself - then we wouldn't need to copy the image data on the RoiTool output. I've not implemented this yet, because it doesn't work when the input is not already wrapped in an ImageData - say, we listen to the assembledOutput of a DetectorAssembler rather than the preview.output.