textbox: add clear() function

Origin commit data
------------------
Commit: 0b504f8c60
Author: vanhofen <vanhofen@gmx.de>
Date: 2016-10-31 (Mon, 31 Oct 2016)

Origin message was:
------------------
- textbox: add clear() function

src/gui/widget/textbox.h

@@ -210,6 +210,7 @@ class CTextBox : public sigc::trackable
 		inline int	getTextMode()			{return m_nMode;};
 		void paint (void);
 		void hide (void);
+		void clear(void);
 		bool clearScreenBuffer();
 		sigc::signal<void> OnAfterRefresh;
 		sigc::signal<void> OnAfterScrollPage;