(bug 43144) Refactor to use i18n for extension tours.

* Make wrapper method, which currently just handles i18n
with jqueryMsg (but could do more later).
* Add all messages needed by test, with qqq documentation.
* Move guiders.currentTour to gt.currentTour since it is not used by
the actual library.
* Make minor phrasing improvements, cleanups, and a TODO.
* Uses temporary hack due to lack of forcontent in ResourceLoader.
Change-Id: Id2f75b5635827c53c080d87a5ff68c767e9b67bf