fix #10199 Non-encodable characters with XeTeX and utf8-plain

fix #10199 Non-encodable characters with XeTeX and utf8-plain

XeTeX with TeX fonts is only safe with ASCII input encoding (see #9740)
and we therefore force "ascii" when exporting with XeTeX and 8-bit TeX-fonts.

However, "utf8-plain" is a "power-user" option, which allows to switch off LyX's
encoding of the LaTeX file:
keep this also for "XeTeX with TeX fonts".

The user is responsible to ensure all characters can be processed and are
correctly shown in the output. The provided test sample shows the problems
with this encoding without special measures (like loading fontspec in the
user-preamble or a document class).