Immediately send placeholder values

[lars-reimann]

Is your feature request related to a problem?

Related to Safe-DS/DSL#1079.

Desired solution

• Remove save_placeholder.
• Add send_placeholder_value(name, value).
• Add report_progress(name).
• Combine program message and placeholder query message. It should still be possible to specify windows for the individual placeholders.
• Remove the map that stores placeholder values. It was a resource hog.

Possible alternatives (optional)

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Additional Context (optional)

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