Make Action icons again include alt text (#13375)

Change-Id: I7b65d8f20936e5e6360dfc4d333e7c6ea3f2bc13

client/src/com/vaadin/client/ui/Action.java

@@ -46,6 +46,7 @@ public abstract class Action implements Command {
         // right now because Actions are recreated every time they are needed
         Icon icon = owner.getClient().getIcon(getIconUrl());
         if (icon != null) {
+            icon.setAlternateText("icon");
             sb.append(icon.getElement().getString());
         }
         sb.append(getCaption());