Library constants as default value

[lukarade]

Is your feature request related to a problem?

Currently there is no way to deal with non literal constants from imported libarys as default values.

Desired solution

Extend default value form shown for @optional, @constant, and @attribute by a radio button: "library constant" and two corresponding input text fields, that are displayed if this button is checked.
First field: "module"
Second field: "declaration"
A corresponding import declaration is generated.

Possible alternatives (optional)

No response

Screenshots (optional)

Additional Context (optional)

Subsumes the following issues:

• required annotation is set, even though a default value exists and is the most used "value"  Library-Analyzer#28
• error_score is required although it has a default value Library-Analyzer#24
• Wrong required annotation for dtype Library-Analyzer#37

[lars-reimann]

Should be done only after #874.