EXT-944 EXT-1026: converted the LLUrlRegistry::findUrl() method to

work on an LLWString instead of a std::string, so that we don't have
to worry about character offsets for variable-length-encoded UTF-8
strings.

This was causing crashes whenever we would try to show a textbox with
a URL and foreign characters (> 1 byte chars). Damn, I suck!