Catch pageup/down and ctrl-home/end at the top level

Make the scroll keys work when the focus is outside the message panel.

src/KeyCode.js

@@ -0,0 +1,32 @@
+/*
+Copyright 2016 OpenMarket Ltd
+
+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.
+*/
+
+/* a selection of key codes, as used in KeyboardEvent.keyCode */
+module.exports = {
+    BACKSPACE: 8,
+    TAB: 9,
+    ENTER: 13,
+    SHIFT: 16,
+    PAGE_UP: 33,
+    PAGE_DOWN: 34,
+    END: 35,
+    HOME: 36,
+    LEFT: 37,
+    UP: 38,
+    RIGHT: 39,
+    DOWN: 40,
+    DELETE: 46,
+};