mod_survey: Improvements

- Update NL translation strings
- Add placeholder text option for short answer fields
- Make the validation messages translateable for short answer fields
(cherry picked from commit 756bb94056dd3586a57fab222427fe112bc6ebf4)
mod_survey: Fix wrong include
(cherry picked from commit 142213f1e0a6fa14a885eecb14f2bee67a24f191)