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.h

@@ -65,6 +65,12 @@ public:
     // and the docktoolbar widgets
     // Similar to how IView
     virtual NavigationView createWidget() = 0;
+
+    // Read and store settings for the widget, created by this factory
+    // and beeing at position position. (The position is important since
+    // a certain type of widget could exist multiple times.)
+    virtual void saveSettings(int position, QWidget *widget);
+    virtual void restoreSettings(int position, QWidget *widget);
 };
 
 } // namespace Core