Invalid GPX XML is exported

[Alexander Borsuk]

& and < characters in <name> and <cmt> tags should be escaped by using & and <.

Or, alternatively, a whole string should be wrapped in CDATA:

  if (s.find_first_of("<&") != std::string::npos)
    writer << "<![CDATA[" << s << "]]>";
  else
    writer << s;

@cyber-toad can you please take a look? It breaks some users' bookmarks.

[Willy]

Suggestion: Implement this for the desc as well.
In waypoints desc is more popular than cmt.
By the gpx standard:

• cmt is suggested for a compact comment.
• desc is suggested for extended description texts.