Fix the online name (to force people to use only one online name).

This prevents people from passing achievements from one online
account to another (once two way synching is in place).

src/states_screens/user_screen.cpp

@@ -111,6 +111,10 @@ void BaseUserScreen::init()
         PlayerProfile *player = PlayerManager::getCurrentPlayer();
         const stringw &online_name = player->getLastOnlineName();
         m_username_tb->setText(online_name);
+        if(online_name.size()>0)
+            m_username_tb->setDeactivated();
+        else
+            m_username_tb->setActivated();
         // Select 'online
         m_online_cb->setState(player->wasOnlineLastTime() ||
                               player->isLoggedIn()          );
@@ -444,6 +448,7 @@ void BaseUserScreen::loginError(const irr::core::stringw & error_message)
     // which allows the player to enter a new password.
     if(player && player->hasSavedSession())
         player->clearSession();
+    player->setLastOnlineName("");
     makeEntryFieldsVisible();
     sfx_manager->quickSound("anvil");
     m_info_widget->setErrorColor();