Export the navigation widget and expose activation of subwidgets.

Reviewed-by: hjk

src/plugins/coreplugin/mainwindow.h

@@ -60,6 +60,7 @@ class MessageManager;
 class MimeDatabase;
 class ModeManager;
 class ProgressManager;
+class NavigationWidget;
 class RightPaneWidget;
 class ScriptManager;
 class SettingsDatabase;
@@ -73,7 +74,6 @@ class ActionManagerPrivate;
 class CoreImpl;
 class FancyTabWidget;
 class GeneralSettings;
-class NavigationWidget;
 class ProgressManagerPrivate;
 class ShortcutSettings;
 class StatusBarManager;