XWIKI-5261 : Password hashes dumpable.

This little fix salt every password. To make sure there is no backward compatibility issue, I treat former unsalted password the same way as before. But when a user changes his password, or when a new user is created, the passwords thus created are automatically salted with a random salt stored in the password.
If we want double salting (or encrypting hashes), it should be easy to add it.
In addition, I hide the password field from the $doc.getValue() method. To be consistent, I did it exactly the same way it is already done in the getValue() method in com.xpn.xwiki.api.Propety.java.  
This will make the whole system terribly slow if the JVM is configured to use the ***really truly absolutely random*** random generator, which is synchronous and usually takes a lot of time to gather enough entropy. Are you sure you really need `SecureRandom` here?