Moved value setter to constructor

[rdotter]

• Changed value function into only getter of the value for a generator class
• Moved setter of __value to constructor (where is should be)

@goetas

[goetas]

I think this channges are not backward compatible, are they?

[rdotter]

@goetas the setter via value is only used by JMS and I hope not ever by a developer :-)
So when you regenerate the models + yml everything keeps working only the generated models differs a bit (less code).

If a developer used the value function for setting this variable for creating model for generating XML it will break because it is no longer accepted by this method. I think its a consistency improvement to do it this way.

[rdotter]

@goetas can you merge this PR? :-)

[goetas]

I'm worried about BC compatibility and more than that, I'm not sure if is a good idea to call __construct on an already initialized object.

[rdotter]

@goetas you are totally right, I thought it would call the construct on creation of the class, but I found you have to use an unserialize_object_constructor.class. I'll look into this next week. Thanks for the feedback.

Maybe only the doc change is required for now, I only dislike the hidden setter :-)