[Improvement] Fix Checkbox mulitple values

[tertek]

Currently only first checkbox value will be rendered. Add support to output multiple comma-separated values.

[tertek]

Injecting multiple checkboxes as form checkboxes values is not possible without breaking the whole module logic.
It is better to resolve this by injecting multiple selections as comma separated string into form text field.

For injecting multiple choice as separated check boxes the user can still use calculated fields in combination with if clauses.