We need the new entry in Settings, near Home directory, named "Default export
directory". It should be possible to set the directory there, which would be
offered by default when exporting to some fileformat, say PDF (could also affect
the SWF of other future export formats).

In our office, users work among their documents. Whenever they finish some event
and they send document outside in paper form, they should export PDF too and
link to it in the issue tracking system. The PDF's are then kept "forever" to
allow later review of what has been sent in order of particular issue. It's
clear, that such "frozen" PDF's shouldn't mess up with "live" documents.

For that case, defining the default export directory would help much.

If user dosen't set the directory, it could gain the value from "Home directory"
entry.

Shouldn't be too hard to implement, is it?