[Feature request] Add `SelectedTimeSpan` `DependencyProperty` and associated events to `TimePicker`

[thargy]

Is your feature request related to a problem? Please describe. See #1285 for more details. Raising here to allow separate tracking.

Describe the solution you'd like Currently, the TimePicker can only be bound to DateTime?. Adding an associated SelectedTimeSpan to sit alongside SelectedTime and making it type TimeSpan? would make it significantly easier to bind the control directly to a TimeSpan?.

Describe alternatives you've considered A type converter can be used to convert to/from DateTime? and TimeSpan?. However, that requires more coding from the consumer.

There is a case to have a dedicated TimeSpanPicker which also supports 'days'.

Additional context N/A