Fixes: Saving and restoring of ProjectTreeWidget states

Task:     Reported on the ML multiple times.
Details:  Add a few methods to make it possible to save and restore
settings. Save those settings to the .ini file. (I'm not sure whether
seesion would be better, let's see what feedback we get.)

src/plugins/coreplugin/inavigationwidgetfactory.cpp

@@ -48,3 +48,13 @@ QKeySequence INavigationWidgetFactory::activationSequence()
     return QKeySequence();
 }
 
+
+void INavigationWidgetFactory::saveSettings(int position, QWidget *widget)
+{
+
+}
+
+void INavigationWidgetFactory::restoreSettings(int position, QWidget *widget)
+{
+
+}