AbstractDateField: open calendar popup on focus

We'd like to see the possibility to open calendar popup on focus event in the future versions of Vaadin.

Possible interface could be this:

 public abstract class AbstractDateField {
      ...
      private boolean openCalendarOnFocus = false;
      ...
      public void setOpenCalendarOnFocus(boolean openCalendarOnFocus) {
          this.openCalendarOnFocus = openCalendarOnFocus;
      }
 }

Or even this (to provide more control to users):

 public abstract class AbstractDateField {
      ...
      public void openCalendar() {
          ...
      }
 }
 ...
 dateField.addFocusListener((FieldEvents.FocusListener) event -> dateField.openCalendar());

Related SO question

Start by locating AbstractDateField and reviewing how focus and the calendar popup are currently handled. Decide which public API and focus behavior should be supported, then define the expected popup behavior and test coverage before considering the feature complete.