Functions to provide xml-valid strings

By removing invalid chars, see http://www.w3.org/TR/xml/#charsets

src/utils/encoding.hpp

@@ -11,6 +11,14 @@ namespace utils
    * Based on http://en.wikipedia.org/wiki/UTF-8#Description
    */
   bool is_valid_utf8(const char* s);
+  /**
+   * Remove all invalid codepoints from the given utf-8-encoded string.
+   * The value returned is a copy of the string, without the removed chars.
+   *
+   * See http://www.w3.org/TR/xml/#charsets for the list of valid characters
+   * in XML.
+   */
+  std::string remove_invalid_xml_chars(const std::string& original);
   /**
    * Convert the given string (encoded is "encoding") into valid utf-8.
    * If some decoding fails, insert an utf-8 placeholder character instead.