ctkPopupWidgetPlugin was not allowing more than 1 child

As soon as the mouse moved outside ctkPopupWidget (in the Designer), the
widget was hidden and no more widgets could be added.

Libs/Widgets/Plugins/ctkPopupWidgetPlugin.cpp

@@ -32,7 +32,11 @@ ctkPopupWidgetPlugin::ctkPopupWidgetPlugin(QObject* pluginParent)
 QWidget *ctkPopupWidgetPlugin::createWidget(QWidget* widgetParent)
 {
   ctkPopupWidget* newWidget = new ctkPopupWidget(widgetParent);
+  // if the widget is a tooltip, it wouldn't accept children
   newWidget->setWindowFlags(0);
+  // if the widget auto hides, it disappear from the workplace and don't allow
+  // children anymore.
+  newWidget->setAutoHide(false);
   return newWidget;
 }