[Enhancement] Add support for CalendarDatePicker on WinUI

[hartez]

Summary

WinUI includes a calendar-style date picking control which would provide an alternative to the DatePicker we currently use: https://docs.microsoft.com/en-us/windows/uwp/design/controls-and-patterns/calendar-date-picker

API Changes

Include a CalendarDatePickerHandler which handles the CalendarDatePicker control. We'd also need to add CalendarDatePickerExtensions to mirror the DatePickerExtensions methods.

Optionally, we could also support setting the Date to null (see #1100, where folks are working on this for the current control). The CalendarDatePicker supports de-selecting the currently selected date, so it works without having to add a "clear" button. At this point, it becomes very tempting to just drop the old control entirely.

Intended Use Case

The current DatePicker is not always the best control for choosing a date. It was designed for compact scenarios, so it doesn't provide as much calendar context and doesn't allow for precision in min/max dates beyond limiting the year segment.

Providing an alternative handler and allowing for users to opt-in to that handler would allow for a potentially richer experience on the Windows platform.

Opting in to the alternate handler might look something like this (in application configuration):

#if WINDOWS
.ConfigureHandlers(handlers => handlers.AddHandler<DatePicker, CalendarDatePickerHandler>())
#endif

Difficulty

Super easy, barely an inconvenience.

[hartez]

#1404