The code blindly expected UTF8_STRING for files.  It did not downgrade
based on source protocol version.  It did not handle hostnames in
text/uri-list data.  It did not specify the source time stamp when
converting the selection.  It did not search the XdndTypeList when
necessary.  It did not ignore sources that specified invalid versions.

While better, this is still not fully conformant.  Hostnames are not
validated and it does not guard against source crashes.

Fixes #968.