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.cpp

@@ -309,6 +309,11 @@ void ModeManager::setFocusToCurrentMode()
     }
 }
 
+void ModeManager::switchToDefaultMode()
+{
+    d->m_modeStack->setCurrentIndex(0);
+}
+
 ModeManager *ModeManager::instance()
 {
     return ModeManagerPrivate::m_instance;