Method to make calendar show on PopupDateField

Originally by stevebor1

There might be times when the calendar popup should be displayed on init and not require the user the explicitly click the button. Also would be good to be able to create a focus listener on the textfield and trigger the calendar to display without the need to explicitly click the button.

Request

#!div style="font-size: 80%"
Code highlighting:
  {{{#!java
PopupDateField datefield  new PopupDateField();
dateField.showCalendar(true);

}}}

Imported from https://dev.vaadin.com/ issue #19852

Start by locating the PopupDateField implementation and inspect how its calendar popup is opened and how focus events are handled. Confirm the intended showCalendar behavior from the issue, then check the existing PopupDateField tests or UI test entry points; done means the calendar can open on initialization or from text-field focus without requiring the button.