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/modemanager.h

@@ -70,6 +70,8 @@ private:
     explicit ModeManager(Internal::MainWindow *mainWindow, Internal::FancyTabWidget *modeStack);
     ~ModeManager();
 
+    static void extensionsInitialized();
+
     static void addMode(IMode *mode);
     static void removeMode(IMode *mode);
     void currentTabAboutToChange(int index);