XPOLL-2:Displayed Xpoll link in the panel of list of Applications

* Add the object XWiki.UIExtensionClass to WebHome
* Add Some French translations
XPOLL-3:The XPoll app French translations are not complete

* Add a translation key in the XPollSheet document.
XPOLL-3:The XPoll app French translations are not complete
* Correction of the translation keys
Merge pull request #1 from zoubir/master

XPOLL-2:Displayed Xpoll link in the panel of list of Applications
Merging Zoubir's pull request, which I will fix later in a subsequent commit.