Use methods for field assignment

[smaugho]

When injections are done, the values are assigned to the injected values automatically, this mechanism makes difficult to support @External annotation for fields. For every injection, a method to assign the variable (or read it) should be done, so the injection can be done @Externally as well (this permits that an @External class inject behavior in the class which is using it also by fields, without needing @ExternalPopulate or @ExternalRecollect, or any different annotation).