Display notifications using the browser's notification feature

This should be generic and the notification preferences UI should be modified so that the user decides. Right now the user decides if he wants to receive email or alerts. We need to add a 3rd option (browser notifications).

More info:

• Integration of the notifications of user mentions into the browser notifications.
• See https://design.xwiki.org/xwiki/bin/view/Proposal/UserMentions#HUC10.IntegrationofbrowserA0notifications