CLuaInstance: Add strFind() and strSub() functions

- These functions are possible substitutes for the
  Lua functions string.find() and string.sub()

- This Lua functions have problems e.g. with the contents
  of some websites.

src/gui/luainstance.h

@@ -195,6 +195,9 @@ private:
 	static int DisplayImage(lua_State *L);
 	static int PlayFile(lua_State *L);
 
+	static int strFind(lua_State *L);
+	static int strSub(lua_State *L);
+
 	void MenuRegister(lua_State *L);
 	static int MenuNew(lua_State *L);
 	static int MenuDelete(lua_State *L);