Merge: add the annotation `readonly` on attributes

`readonly` just generate no setter. So the value must be set trough the initial value.

Basically it is like the Scala `val` keyword.
Note that unlike the Java `final` modifier, the value cannot be set in a constructor (use a private setter instead if you need a restriction on the setter visibility)

~~~.rb
class Toto
   var a = new Array[Int] is readonly
end
var t = new Toto
t.a.add 5 # OK
t.a = new Array[Int] # Error: no `a=` in Toto
~~~

Pull-Request: #604
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc src/modelize_property.nit
index b618676,898366b..96ec598
--- 1/src/modelize_property.nit
--- 2/src/modelize_property.nit
+++ b/src/modelize_property.nit
@@@ -718,14 -706,17 +718,23 @@@ redef class AAttrPropde
                        modelbuilder.mpropdef2npropdef[mreadpropdef] = self
                        mreadpropdef.mdoc = mpropdef.mdoc
  
+                       var atreadonly = self.get_single_annotation("readonly", modelbuilder)
+                       if atreadonly != null then
+                               if n_expr == null then
+                                       modelbuilder.error(atreadonly, "Error: a readonly attribute needs a value")
+                               end
+                               # No setter, so just leave
+                               return
+                       end
+ 
                        var writename = name + "="
                        var nwritable = self.n_writable
 +                      var atwritable = self.get_single_annotation("writable", modelbuilder)
 +                      if atwritable != null then
 +                              if not atwritable.n_args.is_empty then
 +                                      writename = atwritable.arg_as_id(modelbuilder) or else writename
 +                              end
 +                      end
                        var mwriteprop = modelbuilder.try_get_mproperty_by_name(nid2, mclassdef, writename).as(nullable MMethod)
                        var nwkwredef: nullable Token = null
                        if nwritable != null then nwkwredef = nwritable.n_kwredef