pat-validation: Validate a date input which was changed to text.

[thet]

There are cases where a <input type="date"> is changed to <input type="text"> by pat-date-picker:

1. If a browser does not support the date type (should be rare these days),
2. If we want to prevent the native date picker but use the JS one (e.g. for i18n where the browser language is different from the websites's UI language, where we need to support options not available with the native date picker, ...)

The date validation should then still be active. For that we need some changes in pat-validation. E.g. pat-validation validating text inputs with a pat-date-picker class.