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/_locales/en/messages.json b/src/_locales/en/messages.json
index 6366c58..1f950f3 100644
--- a/src/_locales/en/messages.json
+++ b/src/_locales/en/messages.json
@@ -54,8 +54,8 @@
 		"description": "Option which defines where should the translations be shown (in this case, each translation will be opened in the same tab used for the previous translation, or will open a new tab if there aren't any translation tabs open)."
 	},
 	"options_tabsoption_3": {
-		"message": "Open Google Translate in a panel (you need to activate <a href='chrome://flags/#enable-panels' id='panelsflag'>--enable-panels</a> flag). <sup style='color:red; font-weight: bold;'> EXPERIMENTAL</sup>",
-		"description": "Option which defines where should the translations be shown (in this case, each translation will be opened in a new panel)."
+		"message": "Open Google Translate in a popup.",
+		"description": "Option which defines where should the translations be shown (in this case, each translation will be opened in a new popup)."
 	},
 	"options_savebutton": {
 		"message": "Save",