Fix plaintext output of dashes (bug #3647)

Fix plaintext output of dashes (bug #3647)

Previously, consecutive dashes in .lyx files were combined to endash and emdash
in some cases, and in other cases they were output as is. This made the code
complicated, and resulted in inconsitencies ((bug #3647).
Now, a dash in a .lyx file is always a dash in the output, for all flavours.
The special handling is moved to the input side, so that you still get an
endash if you type two hyphens. If needed, this can be changed or made
customizable without the need to update the file format again. Many thanks
for the fruitful mailing list dicsussion, which contributed significantly to
the final version.