Refine DatePicker API and impement DatePickerStyle support [Gtk][Mac]

This PR adds DatePickerStyle support to Gtk and Mac backends, and includes many fixes in the backend implementations.

* GTK:
  * fix component selection
  * fix (keep) seconds when other component chages
  * fix keyboard input (incl. arrow keys and separators)
  * fix mouse selection
  * add internationalization support
    * use localized separators
    * use localized component order
    * no 12/24 support
  * add DatePickerStyle support (show only relevant components)
  * add clipboard support
* MAC:
  * add DatePickerStyle support
  * fix DatePicker timezone conversion

For WPF we still need a useful DateTimePicker component. Maybe something external from NuGet?