Fix save list, delete save, exit without saving, and blank username on Windows64 (#539)

* Fix world save rename not applying new name

KeyboardCompleteWorldNameCallback had no _WINDOWS64 branch, so the
typed name was validated then silently discarded on every rename attempt.

Write the new name to a worldname.txt sidecar file next to the save
(Windows64\GameHDD\{folder}\worldname.txt) and update the in-memory
display name immediately. ReadLevelNameFromSaveFile now checks for this
sidecar first so renamed saves persist correctly across restarts.

* Fixed gamertag being blank upon renaming and re-joining a save

* Save deletion fix, exiting without saving fix

* Add native in-game keyboard UI for world naming and renaming

Minecraft.Client/MinecraftServer.cpp

@@ -1230,7 +1230,11 @@ bool MinecraftServer::loadLevel(LevelStorageSource *storageSource, const wstring
 
 	if( levels[0]->isNew || levels[1]->isNew || levels[2]->isNew )
 	{
+#ifndef _WINDOWS64
+		// On Windows64 we skip the automatic initial save so that choosing
+		// "Exit without saving" on a new world does not leave an orphaned save folder.
 		levels[0]->saveToDisc(mcprogress, false);
+#endif
 	}
 
 	if( s_bServerHalted || !g_NetworkManager.IsInSession() ) return false;