Fix crash on close in sidebar/bookmarks view

Explicitly close all views of the sidebar _before_ the core is
destructed. E.g. the bookmarks view tries to access the core in
it's destructor.

src/plugins/coreplugin/navigationwidget.h

@@ -92,6 +92,8 @@ public:
     void saveSettings(QSettings *settings);
     void restoreSettings(QSettings *settings);
 
+    void closeSubWidgets();
+
     bool isShown() const;
     void setShown(bool b);