Added save indicator in the options page in Gecko

Firefox doesn't allow extensions to close the options page via
window.close, so this commit adds a "Saved" message each time the
options are saved.

In Chromium, the behavior continues to be the same as before (the
options page is closed after saving the changes).

Change-Id: I4f313f382b527e205ba1dc9976af53cb13a0cba6
diff --git a/src/options.html b/src/options.html
index 8edaea2..3df88c5 100644
--- a/src/options.html
+++ b/src/options.html
@@ -13,13 +13,14 @@
     <h4 data-i18n="enhancements"></h4>
     <p>
       <input type="checkbox" id="fixedtoolbar"> <label for="fixedtoolbar" data-i18n="fixedtoolbar"></label><br>
-      <input type="checkbox" id="redirect"> <label for="redirect" data-i18n="redirect"></label> <span style="color: gray;" data-i18n="experimental_label"></span><br>
+      <input type="checkbox" id="redirect"> <label for="redirect" data-i18n="redirect"></label> <span class="experimental-label" data-i18n="experimental_label"></span><br>
       <input type="checkbox" id="history"> <label for="history" data-i18n="history"></label><br>
-      <input type="checkbox" id="loaddrafts"> <label for="loaddrafts" data-i18n="loaddrafts"></label> <span style="color: gray;" data-i18n="experimental_label"></span><br>
+      <input type="checkbox" id="loaddrafts"> <label for="loaddrafts" data-i18n="loaddrafts"></label> <span class="experimental-label" data-i18n="experimental_label"></span><br>
       <input type="checkbox" id="increasecontrast"> <label for="increasecontrast" data-i18n="increasecontrast"></label><br>
       <input type="checkbox" id="stickysidebarheaders"> <label for="stickysidebarheaders" data-i18n="stickysidebarheaders"></label><br>
     </p>
-    <p style="text-align: center;"><button id="save" data-i18n="save"></button></p>
+    <p class="actions"><button id="save" data-i18n="save"></button></p>
+    <div id="save-indicator"></div>
     <script src="options.js"></script>
   </body>
 </html>