Better handling of different timezones for date properties

Currently, dates in XWiki do not take into consideration the user timezone. One possible improvement could consist of:

• adding a new property in the user profile where the users can set their timezone; empty timezone = server timezone
• modifying the save and display function of date properties to make the transformation to the user timezone
• adding an API to translate the dates to the user's timezone in applications that need to work directly with the stored value of the date property