Add a clear button for the DateTimePicker

[jcgueriaud1]

Describe your motivation

The components like DatePicker and TimePicker have a clear button functionality.
But the DateTimePicker does not have a clear button.

Describe the solution you'd like

I would like a clear button that clears the field.
Maybe something like this:

Describe alternatives you've considered

One alternative solution is to set the clear button on the internal TimePicker and DatePicker.
It doesn't require a new design but it doesn't properly clear since it clear only half of the value.

mainField.getChildren()
        .filter(c -> c instanceof HasClearButton)
        .map( c -> (HasClearButton) c)
        .forEach(d -> d.setClearButtonVisible(true));

We could also create a external button that clears the 2 fields.

Additional context

No response

[rolfsmeds]

One question that comes to mind here is what pressing Esc in either subfield should do. In other fields it acts as a shortcut to the clear action, clearing the field's value entirely. Should it in this case clear just the currently focused subfield, or both subfields?