Fix output of en- and em-dashes with TeX fonts

Fix output of en- and em-dashes with TeX fonts

This commit fixes the regression introduced in 2.2 about the
output of en- and em-dashes. In 2.2 en- and em-dashes are output as
the \textendash and \textemdash macros when using TeX fonts, causing
changed output in old documents and also bugs (for example, #10490).

Now documents produced with older versions work again as intended,
while documents produced with 2.2 can be made to produce the exact
same output by simply checking "Don't use ligatures for en-and
em-dashes" in Document->Settings->Fonts.

When exporting documents using TeX fonts to earlier versions, in order
to avoid changed output, a zero-width space character is inserted after
each en/em-dash if dash ligatures are allowed. These characters are
removed when reloading  documents with 2.3, so that they don't accumulate.