[DatePicker] Preserve time component of Date

[at0g]

Description

Whenever a Date is chosen in the DatePicker, it defaults the time to midnight.
While this behaviour may be desirable in some situations, it's a nuisance when trying to work with both a DatePicker and a TimePicker (seeing as there is no unified DateTimePicker).

A couple of points:

• The Date object contains both the calendar date components along with time/tz components.
• The DatePicker operates on the calendar date components of the Date object, it probably should not mess with the time (at the very least, this should be a configuration option)
• The TimePicker does not reset the calendar part of the date back to 1/1/1970 when a time is chosen

[oliviertassinari]

Closing for #4787