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/Extrax64Stubs.cpp

@@ -329,6 +329,10 @@ void IQNet::EndGame()
 		m_player[i].m_gamertag[0] = 0;
 		m_player[i].SetCustomDataValue(0);
 	}
+	// Restore local player 0's gamertag so re-joining works correctly
+	extern wchar_t g_Win64UsernameW[17];
+	m_player[0].m_isHostPlayer = true;
+	wcscpy_s(m_player[0].m_gamertag, 32, g_Win64UsernameW);
 }
 
 DWORD MinecraftDynamicConfigurations::GetTrialTime() { return DYNAMIC_CONFIG_DEFAULT_TRIAL_TIME; }