helpers: add trim methodes for left and right trim

Origin commit data
------------------
Commit: 5e1b9aaa4d
Author: Thilo Graf <dbt@novatux.de>
Date: 2019-03-28 (Thu, 28 Mar 2019)

src/system/helpers.cpp

@@ -613,6 +613,18 @@ std::string trim(std::string &str, const std::string &trimChars /*= " \n\r\t"*/)
 	return result.erase(0, result.find_first_not_of(trimChars));
 }
 
+std::string ltrim(std::string& str, const std::string& trimChars)
+{
+	str.erase(0, str.find_first_not_of(trimChars));
+	return str;
+}
+
+std::string rtrim(std::string& str, const std::string& trimChars)
+{
+    str.erase(str.find_last_not_of(trimChars) + 1);
+    return str;
+}
+
 std::string cutString(const std::string str, int msgFont, const int width)
 {
 	Font *msgFont_ = g_Font[msgFont];