Add MessageOfTheDay extension point for plugins

Allow plugins to contribute messages to the UI during initial page
load. Gerrit displays the messages in a butter bar at the top of
the page and allows users to hide them with the "Dismiss" button.

Messages are hidden per-browser using a cookie named after the
message id, set to expire at a date supplied by the server. After
this date the same message could redisplay if the server sends the
same message again.

Change-Id: I0bcca845f501cbeb8c31356fff398c3adb43099a

gerrit-gwtui/src/main/java/com/google/gerrit/client/MessageOfTheDayBar.java

@@ -0,0 +1,89 @@
+// Copyright (C) 2014 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.client;
+
+import com.google.gerrit.common.data.HostPageData;
+import com.google.gwt.core.client.GWT;
+import com.google.gwt.event.dom.client.ClickEvent;
+import com.google.gwt.uibinder.client.UiBinder;
+import com.google.gwt.uibinder.client.UiField;
+import com.google.gwt.uibinder.client.UiHandler;
+import com.google.gwt.user.client.Cookies;
+import com.google.gwt.user.client.ui.Anchor;
+import com.google.gwt.user.client.ui.Composite;
+import com.google.gwt.user.client.ui.HTML;
+import com.google.gwt.user.client.ui.HTMLPanel;
+import com.google.gwt.user.client.ui.RootPanel;
+import com.google.gwtexpui.safehtml.client.SafeHtml;
+import com.google.gwtexpui.safehtml.client.SafeHtmlBuilder;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/** Displays pending messages from the server. */
+class MessageOfTheDayBar extends Composite {
+  interface Binder extends UiBinder<HTMLPanel, MessageOfTheDayBar> {}
+  private static final Binder uiBinder = GWT.create(Binder.class);
+
+  private final List<HostPageData.Message> motd;
+  @UiField HTML message;
+  @UiField Anchor dismiss;
+
+  MessageOfTheDayBar(List<HostPageData.Message> motd) {
+    this.motd = filter(motd);
+    initWidget(uiBinder.createAndBindUi(this));
+
+    SafeHtmlBuilder b = new SafeHtmlBuilder();
+    if (motd.size() == 1) {
+      b.append(SafeHtml.asis(motd.get(0).html));
+    } else {
+      for (HostPageData.Message m : motd) {
+        b.openDiv();
+        b.append(SafeHtml.asis(m.html));
+        b.closeDiv();
+      }
+    }
+    message.setHTML(b);
+  }
+
+  void show() {
+    if (!motd.isEmpty()) {
+      RootPanel.get().add(this);
+    }
+  }
+
+  @UiHandler("dismiss")
+  void onDismiss(ClickEvent e) {
+    removeFromParent();
+
+    for (HostPageData.Message m : motd) {
+      Cookies.setCookie(cookieName(m), "1", m.redisplay);
+    }
+  }
+
+  private static List<HostPageData.Message> filter(List<HostPageData.Message> in) {
+    List<HostPageData.Message> show = new ArrayList<HostPageData.Message>();
+    for (HostPageData.Message m : in) {
+      if (Cookies.getCookie(cookieName(m)) == null) {
+        show.add(m);
+      }
+    }
+    return show;
+  }
+
+  private static String cookieName(HostPageData.Message m) {
+    return "msg-" + m.id;
+  }
+}