ui: date-picker - translate placeholder internally as before

[SuperITMan]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

Currently, the placeholder has to be translated by the the component which is calling stark-date-picker --> <stark-date-picker [placeholder]="'MY_TRANSLATION_KEY' | translate".

Previously, it was handled by the date-picker itself.

Expected behavior

We should handle the translation again in the date-picker component.

What is the motivation / use case for changing the behavior?

Improve the developer's life 😊