Optional prefix for generated labels and placeholders

[veith]

When running muSpec2Spec the generated labels and placeholders are
package.type.fiel.label and package.type.fiel.placeholder

It would be nice when we could set a prefix in the .furo file to avoid collisions.

The prefix should also applied to the generated enum option labels from genEsModule.
The prefix should also applied to the required.is.message