Fixed: CPDatePicker firstResponder issue

Previously, the textual datePicker didn't work because it couldn't become the firstResponder.
This PR fixed another issue as well. Previously it wasn't possible to modify a date when moving only with tab and when the datePicker came firstResponder with tabulation.
Merge pull request #2126 from Dogild/DatePickerTabIssue

Fixed: CPDatePicker firstResponder issue