Please describe the problem:
Defining the handler before connecting to the dynamic signal compiles to wrong c code.


Steps to reproduce:
using GLib;

class App: Object {
   void handler(DBus.Object sender){
     message("huh, the handler is never invoked because of the wrong order of the parameters");
   }
   void run(){
      DBus.Connection conn;
      dynamic DBus.Object obj = conn.get_object("abc", "d", "IF");
      obj.someSignal += handler;
      obj.someSignal += handler2;
   }
   void handler2(DBus.Object sender){
     message("The handler2 is OK");
   }
}


Actual results:
static void app_handler (App* self, DBusGProxy* sender);
static void app_handler2 (DBusGProxy* sender, App* self);


Expected results:
static void app_handler (App* self, DBusGProxy* sender);
static void app_handler2 (App* self, DBusGProxy* sender);

and proper create 'signal_handler_wrapper' for dynamic signals based on the prototype of the handler functions(as a contrast, for static signals the wrapper are based on the definition of the signal).

Does this happen every time?
Yes

Other information:
Not sure how to start the patch. 

plan:
move the code in Vala.CCodeAssignmentBinding.generate_signal_handler_wrapper to
virtual Vala.Signal.generate_signal_handler_wrapper(Vala.Method m);
and implentment 
override Vala.DyanmicSignal.generate_signal_handler_wrapper(Vala.Method m);

Does it make sense?