Fix handling of files in folders with " in name

On UNIX systems, quote characters are permitted in filenames, but
weren't escaped properly in Poedit.  This conflicted with Poedit's
quoting of all file arguments (which was done to handle _spaces_ in
filenames).

Add a dedicated function for quoting arguments and let it escape " as \"
on UNIX.  This is still not a proper fix, it would be better to avoid
this whole mess by simply passing argv array to wxExecute in the first
place.  That's a bit too risky for 1.6, though, so it will have to be
fixed on master.