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/navigationwidget.h

@@ -84,7 +84,7 @@ public:
     ~NavigationWidget();
 
     void saveSettings(QSettings *settings);
-    void readSettings(QSettings *settings);
+    void restoreSettings(QSettings *settings);
 
     bool isShown() const;
     void setShown(bool b);
@@ -133,6 +133,9 @@ public:
     void setFactory(const QString &name);
     void setFocusWidget();
 
+    void saveSettings(int position);
+    void restoreSettings(int position);
+
     Core::ICommand *command(const QString &title) const;
 
 signals: