Add aria-modal attribute and role='dialog' for modal Window (#10447)

Fixes #10424

client/src/main/java/com/vaadin/client/ui/VWindow.java

                     && nativeEvent.getKeyCode() == KeyCodes.KEY_TAB
                     && nativeEvent.getShiftKey()) {
                 nativeEvent.preventDefault();
-                FocusUtil.focusOnLastFocusableElement(this.getElement());
+                FocusUtil.focusOnLastFocusableElement(getElement());
             }
             if (nativeEvent.getEventTarget().cast() == topTabStop
                     && nativeEvent.getKeyCode() == KeyCodes.KEY_BACKSPACE) {
                     && nativeEvent.getKeyCode() == KeyCodes.KEY_TAB
                     && !nativeEvent.getShiftKey()) {
                 nativeEvent.preventDefault();
-                FocusUtil.focusOnFirstFocusableElement(this.getElement());
+                FocusUtil.focusOnFirstFocusableElement(getElement());
             }
             if (nativeEvent.getEventTarget().cast() == bottomTabStop
                     && nativeEvent.getKeyCode() == KeyCodes.KEY_BACKSPACE) {
     public void setVaadinModality(boolean modality) {
         vaadinModality = modality;
         if (vaadinModality) {
+            getElement().setAttribute("aria-modal", "true");
+            Roles.getDialogRole().set(getElement());
             if (isAttached()) {
                 showModalityCurtain();
             }
             addTabBlockHandlers();
             deferOrdering();
         } else {
+            getElement().removeAttribute("aria-modal");
+            Roles.getDialogRole().remove(getElement());
             if (modalityCurtain != null) {
                 if (isAttached()) {
                     hideModalityCurtain();

uitest/src/test/java/com/vaadin/tests/components/window/ModalWindowFocusTest.java

  */
 package com.vaadin.tests.components.window;
 
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 
 import java.io.IOException;
 
     }
 
+    @Test
+    public void verifyAriaModalAndRoleAttributes() {
+        waitForElementPresent(By.id("firstButton"));
+        WebElement button = findElement(By.id("firstButton"));
+        button.click();
+
+        waitForElementPresent(By.className("v-window"));
+        WebElement windowElement = findElement(By.className("v-window"));
+        String ariaModal = windowElement.getAttribute("aria-modal");
+        assertEquals("true", ariaModal);
+        String role = windowElement.getAttribute("role");
+        assertEquals("dialog", role);
+
+    }
+
     private void pressEscAndWait() {
         new Actions(driver).sendKeys(Keys.ESCAPE).build().perform();
         sleep(100);