Change panel references to popup

Panels have been deprecated for a long time, and when creating them a
popup is created instead, so this change introduces the following:

- Instead of trying to create a panel, create a popup directly
- Change all i18n strings so "popups" are mentioned instead of "panels"
- Change the |uniquetab| option so it is now set to "popup" instead of
  "panel", while being backwards-compatible with the "panel" value.
- Change the default |uniquetab| option to "popup", because users seem
  to like this option the most (impression from the CWS reviews).

Fixes: #13
Change-Id: I67aea8168013dd4b77d4ea4229179bcd5145cb6d
diff --git a/src/options.html b/src/options.html
index ef2efc5..0d40a36 100644
--- a/src/options.html
+++ b/src/options.html
@@ -29,7 +29,7 @@
   <div id="otheroptions">
     <p><input type="radio" name="uniquetab" id="varioustabs" value=""> <label id="varioustabs_label" for="varioustabs" data-i18n="tabsoption_1"></label></p>
     <p><input type="radio" name="uniquetab" id="uniquetab" value="yep"> <label id="uniquetab_label" for="uniquetab" data-i18n="tabsoption_2"></label></p>
-    <p><input type="radio" name="uniquetab" id="panel" value="panel"> <label id="panel_label" for="panel" data-i18n="tabsoption_3"></label></p>
+    <p><input type="radio" name="uniquetab" id="popup" value="popup"> <label id="popup_label" for="popup" data-i18n="tabsoption_3"></label></p>
   </div>
 
   <button id="save" data-i18n="savebutton"></button>