Fix encoding for copying LaTeX from clipboard

Fix encoding for copying LaTeX from clipboard

If we call tex2lyx on a temporary file created from the clipboard, the
file is always in utf8 encoding, without any temporary changes, even if it
contains encoding changing LaTeX commands. Therefore, we must tell tex2lyx
to use a fixed utf8 encoding for the whole file, and this is done using the
new latexclipboard format. Previously, tex2lyx thought the encoding was
latin1.
As a side effect, the -e option is now also documented in the man page.