Added "joins" for related classes.

[nicar]

Can save and retrieve related classes for parent class.
Raw version with commented out debug code, not fully tested!

Added "joins" property.

• Naming convention for forms: class name prepended to fields name, like
  <input name="Alias.field">
• Property input must be JSON notification
  Example:

&joins=`[
 {"CommentImages" : {"type" : "file" , "req" : "false"} },
 {"CommentThumbnail" : {"type" : "file"} },
 ["SingleClass"]
]`

required not yet implemented

Also checks for files and empty file fields; can be used with formit2file hook (file data has to be passed e.g. by $hook->setValue($sfile,$jsonData))