Create WXSendContextMenuEvent() helper function

Put common code from all the different ports into it.

This is not very useful right now, but it will allow to change this
function once, instead of applying the same change to all ports, in the
upcoming commit.

src/osx/window_osx.cpp

@@ -2338,20 +2338,7 @@ void wxWindowMac::OnMouseEvent( wxMouseEvent &event )
 {
     if ( event.GetEventType() == wxEVT_RIGHT_DOWN )
     {
-        // copied from wxGTK : CS
-        // VZ: shouldn't we move this to base class then?
-
-        // generate a "context menu" event: this is similar to wxEVT_RIGHT_DOWN
-        // except that:
-        //
-        // (a) it's a command event and so is propagated to the parent
-        // (b) under MSW it can be generated from kbd too
-        // (c) it uses screen coords (because of (a))
-        wxContextMenuEvent evtCtx(wxEVT_CONTEXT_MENU,
-                                  this->GetId(),
-                                  this->ClientToScreen(event.GetPosition()));
-        evtCtx.SetEventObject(this);
-        if ( ! HandleWindowEvent(evtCtx) )
+        if ( !WXSendContextMenuEvent(ClientToScreen(event.GetPosition())) )
             event.Skip() ;
     }
     else