Make document_language non-mutable

The many occurances of the mutable keyword in LyX sources make it quite
difficult to understand what is really const and what not, and to debug
multithreading memory problems. In this particular case it is not needed
at all to have a mutable member, so make it non-mutable.
Remove unneeded mutable keywords
use enchant exception message accessor
Show export message also for non-threaded export
Fix viewer file paths on windows (bug #8892)

Some applications do not accept forward slashes, so call external viewers
and editors with backward slashes which is the usual convention under windows.
tex2lyx: support for the libertine fonts

- all libertine font variants are now supported
tex2lyx/Preamble.cpp: whitespace fix

- revert unnecessary change from previous commit