Fix bug #8468: Wrong import of UTF8 CJK.

Fix bug #8468: Wrong import of UTF8 CJK.

Actually, the test case showed several problems:
- ERT insets did use layout "Standard", not "Plain Layout"
- The font scale was read correctly, but tex2lyx claimed that it did ignore
  the option "scaled=0.95"
- If a third argument of the CJK environment was given, it caused the whole
  environment to be put in ERT with a broken encoding. This is now fixed for
  the bug test case by using the \font_cjk header variable, but the encoding
  problem still exists for unsupported encodings. I'll file a separate bug
  for that.
- The CJKutf8 package was not handled in the preamble parsing. Therefore the
  chinese comment in the preamble was read with a wrong encoding, and guessing
  the document language did not work.
The new file CJKutf8.tex was created by copying and modifying CJK.tex, but
unfortunately it is impossible to tell git to inherit the history of CJK.tex
for the new file (search the web for git svn copy if you want to know details).