Add a way to remove start and end date from DateTextFieldConfig

[jarekCC]

Hi.
I'm trying to remove StartDate or EndDate key from DateTextFieldConfig. Unfortunately current API doesn't allow such operation. I found one way to do this :

config.remove(new Key<>("startDate", null));

but I treat it as a workaround rather then a good solution.

I've noticed few possibilities to make it easier and prettier:

1. when null is passed to 'withStartDate' or 'withEndDate' then remove key from config rather than set it to today's date
2. provide some 'without' methods to remove values from config
3. or just simply change keys visibility from private to public - than something like this:

config.remove(DateTextFieldConfig.StartDate);

can be used - which is in my opinion much better than now.