Avoid focus loss event when wxComboBox popup is opened in wxQt

Prevent wxComboBox from generating a wxEVT_KILL_FOCUS event when the
user opens the drop-down list under wxQt, as logically the drop-down is
part of wxComboBox, even if it's a separate window at Qt level.

Closes https://github.com/wxWidgets/wxWidgets/pull/1377

include/wx/qt/combobox.h

@@ -80,6 +80,7 @@ public:
     virtual void Popup();
     virtual void Dismiss();
 
+    virtual bool QtHandleFocusEvent(QWidget *handler, QFocusEvent *event) wxOVERRIDE;
 protected:
 
     // From wxTextEntry: