Use QXml for export engine

Change import/export engine filter to Qt format
Qt doesn't add extension in its file dialog
Fix a bug : op was not correctly set by default in OnOperationModified (GridAccount)

src/model/import/XMLImportEngine.cpp

@@ -263,8 +263,8 @@ XMLImportEngine::XMLImportEngine()
 {
     KissCount::RegisterImportEngine(this);
 
-    _shortExt = "xml";
-    _longExt = _("KissCount xml files (*.xml)|*.xml");
+    _shortExt = ".xml";
+    _longExt = _("KissCount xml files (*.xml)");
 
     _sax.startElement = XmlStartElement ;
 }