[Feature request] Emit selected event on DatePicker

[lucianholt97]

What problem does this feature solve?
Listen/ react to first click when selecting a range.
This would allow to create an optional range picker without needing the second click when selecting a single day.
Alternatively the input event could be fired on both clicks with [Date, null] on the first click.

What does the proposed API look like?
The selected event only needs to be bubbled from the Panel components.

[mengxiong10]

same as #429