Ensure always having a mode

Switch to Edit Mode once we constructed it in Core::init(). Switch to a
the Welcome Mode in Core::extensionsInitialized. Also works

src/plugins/coreplugin/modemanager.h

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