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/qt/window.cpp

@@ -1619,15 +1619,11 @@ bool wxWindowQt::QtHandleCloseEvent ( QWidget *handler, QCloseEvent *WXUNUSED( e
 
 bool wxWindowQt::QtHandleContextMenuEvent ( QWidget *WXUNUSED( handler ), QContextMenuEvent *event )
 {
-    wxContextMenuEvent e( wxEVT_CONTEXT_MENU, GetId() );
-    e.SetPosition(
+    const wxPoint pos =
         event->reason() == QContextMenuEvent::Keyboard
             ? wxDefaultPosition
-            : wxQtConvertPoint( event->globalPos() )
-    );
-    e.SetEventObject(this);
-
-    return ProcessWindowEvent( e );
+            : wxQtConvertPoint( event->globalPos() );
+    return WXSendContextMenuEvent(pos);
 }
 
 bool wxWindowQt::QtHandleFocusEvent ( QWidget *WXUNUSED( handler ), QFocusEvent *event )