Core: Force and rely on mode creation before extensionsInitialized

cf7f898db3 broke initial mode display due to the then-intentional
delay in registration. Now change the approach: Only record
the modes and the last requested start mode and do all remaining
setup triggered from MainWindow::extensionsInitialized.

This changes behavior insofar as only the last requested mode
on startup (typical Welcome, if Welcome not loaded, Edit) will
be activated.

Change-Id: I62b28342c347938b001c6d3be6a076c5a69b560b
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/mainwindow.cpp

@@ -327,6 +327,8 @@ void MainWindow::extensionsInitialized()
     m_leftNavigationWidget->setFactories(INavigationWidgetFactory::allNavigationFactories());
     m_rightNavigationWidget->setFactories(INavigationWidgetFactory::allNavigationFactories());
 
+    ModeManager::extensionsInitialized();
+
     readSettings();
     updateContext();