Please add support for pulling screenshots from a connected instrument

[makerprobe]

When the HAL supports pulling screenshots, it would be great to be able to trigger that in ngscopeclient. Ideally, I'd like to be able to

• navigate to a connected instrument that supports screenshots
• grab one
• view it
• save it as png

Related issue in scopehal: https://github.com/glscopeclient/scopehal/issues/738