Preloading starts despite having resources that aren't fully loaded

[Fulgen301]

C4Game::InitGameFirstPart indirectly calls C4Network2::RetrieveRes as part of the normal loading processs. This is intended behaviour - for example, C4Network2::RetrieveScenario also performs other steps than just loading the scenario.

In rare occasions, however, C4Network2::RetrieveRes then displays the "Wait for resource" dialog that is known from runtime join or starting a game before all resources have been loaded. This should not happen since preloading is only supposed to start after all resources are already loaded. This also causes crashes like #68 since opening a GUI dialog is looping the event loop, which in case of preloading doesn't happen in the main thread.