Bug: ascii export

[Dekel Tsur]

When doing "lyx --export text", the result is incorrect as the section
counters do not appear in the output (as we didn't call to UpdateCounters).
To fix the problem, the methods UpdateCounters and SetCounter should be
moved from LyXText to Buffer (actually, LyXText::UpdateCounters should
remain and do a call to Buffer::UpdateCounters, and also do the paragraph
rebreaking). Should I create a patch for this ? (or are there better
suggestions ?)