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/Windows64/KeyboardMouseInput.h

@@ -84,6 +84,11 @@ public:
 	void SetScreenCursorHidden(bool hidden) { m_screenWantsCursorHidden = hidden; }
 	bool IsScreenCursorHidden() const { return m_screenWantsCursorHidden; }
 
+	// Text input: buffer characters typed while the native keyboard scene is open
+	void OnChar(wchar_t c);
+	bool ConsumeChar(wchar_t &outChar);
+	void ClearCharBuffer();
+
 	float GetMoveX() const;
 	float GetMoveY() const;
 
@@ -129,6 +134,11 @@ private:
 	bool m_kbmActive;
 
 	bool m_screenWantsCursorHidden;
+
+	static const int CHAR_BUFFER_SIZE = 32;
+	wchar_t m_charBuffer[CHAR_BUFFER_SIZE];
+	int m_charBufferHead;
+	int m_charBufferTail;
 };
 
 extern KeyboardMouseInput g_KBMInput;