Add support for adding other body part types in XWiki.Mail templates and more

Right now the XWiki.Mail xclass has 2 fields for acting on the body part: "html" and "text".

What if the user wanted to add a calendar event in the template?
What if the user wanted to define the list of recipients in the template?
What if the user had the need to set some special mail headers in the template?

The proposed solution is to add a new "script" textarea containing wiki syntax and which would have a "message" script variable bound to the MimeMessage instance used when calling the template with addPart(). This allows the script to virtually configure anything it wants, while still preserving the existing fields for the simple use cases (text and html).