Fixed: CPDatePicker freeze when hidden

Previously the CPDatePicker freezed when trying to hide it when it was the firstResponder.
There was an intern problem in the CPDatePickerTextField about the nextKeyView. That's why there was a boundless loop and the freeze.
This PR also fixed a big about the previous and next textField in the CPDatePicker.
It also fixed a bug about the CPDatePicker when it becomes firstResponder.

Fixes #1909
Merge pull request #1916 from Dogild/DatePickerHidden

Fixed: CPDatePicker freeze when hidden

Reviewed