Make programmatic screenshot taking more useful

[Wumpf]

#12405 introduced an experimental API for taking screenshots. It's already quite powerful but there's still a lot left on the table to make it more useful:

• Experimental screenshot API puts Viewer into focus #12481
• allow blocking on when the screenshot is done
• improve send_blueprint ergonomics a little bit
• provide an example screenshotting out a camera path (needs above items)
• not directly related but makes this much more powerful: allow setting time cursor from viewer client
• work with a windows-less (headless) viewer
• address views by name rather than id (best effort?)

Related issues:

• Feature Request: Autocapture Screenshots from Predefined Virtual Camera Poses #4219