Add -copyfiles command line option to tex2lyx

Add -copyfiles command line option to tex2lyx

If this option is given, included files will be copied to the output directory.
Also -roundtrip is now allowed with given output file.
-copyfiles is useful if you want to ensure that no file (not even an included
one) is overwritten by a subsequent export from LyX. Both changes are needed
for unit tests that do not write to the source directory.