Add a Panels entry point (in the Applications Panel)

There are 2 aspects related to panels:

• Ability to see existing panels + edit them or create new panels (this is currently Panels.WebHome).
• Ability to configure the wiki to decide which panels are visible by default (this is done in the Administration)

This issue is about an entry point for listing existing panels, editing them or adding new panels.

I believe that developers should be able to see that. However at the moment there's no notion of a developer profile in XWiki (we only have the notion of Simple vs Advanced users). Right now I'm proposing to only show the Panels entry in the Application Panels when the user has Admin rights. We'll need to fix this in the future by having some Developer preset in the user's profile (see http://design.xwiki.org/xwiki/bin/view/Improvements/UserRoles )