Modes: Do the mode switching again like it used to be

It's more complicated then expected, to be detangled another day

src/plugins/coreplugin/modemanager.h

@@ -66,8 +66,6 @@ public:
     void addProjectSelector(QAction *action);
     void addWidget(QWidget *widget);
 
-    void switchToDefaultMode();
-
 signals:
     void currentModeAboutToChange(Core::IMode *mode);