Currently there is pcbnew.ExportSpecctraDSN() but it needs a pcbnew
frame set which makes it unusable outside of the scripting window or a
plugin.
This refactors actual exporting logic into separate helper and adds an
overload to ExportSpecctraDSN(board, filename) that doesn't need pcbnew
window.
They don't define a KiCad string class, so the header file name was
somewhat misleading. But the fact that they didn't match definitely
made coding more difficult.