Add the ability to set the default focus element when modal is opened.

[mP1]

Is your feature request related to a problem? Please describe.
I would like the ability for code to give the element that should be given focus when a Dialog opens, eg:

Dialog dialog = this.dialog
dialog.setFocusElement( this.patternTextBox.element() );
dialog.open()

Describe the solution you'd like
Currently Dialog.open discovers the first focusable element and gives focus to that.

AbstractDialog.initFocusElements is used to discover the first focusable element and stores in this a field...

    NodeList<Element> elementNodeList =
        element.querySelectorAll(
            "a[href], area[href], input:not([disabled]), select:not([disabled]), textarea:not([disabled]), button:not([disabled]), [tabindex=\"0\"]");
    List<Element> elements = elementNodeList.asList();

    if (elements.size() > 0) {
      focusElements = elements;
      firstFocusElement = focusElements.get(0);
      lastFocusElement = elements.get(elements.size() - 1);
    } else {
      lastFocusElement = contentElement.element();
    }