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.css b/src/options.css
index a6fb4de..69a410c 100644
--- a/src/options.css
+++ b/src/options.css
@@ -2,3 +2,17 @@
   cursor: help;
   border-bottom: dashed 1px gray;
 }
+
+.experimental-label {
+  color: gray;
+}
+
+.actions {
+  text-align: center;
+}
+
+#save-indicator {
+  text-align: center;
+  margin-bottom: 16px;
+  color: green;
+}