Export the last selected GUI configuration automatically.

Avoids the annoying save, accept, export, accept pattern. Now you just
need to save. This is the first half of fixing issue #837. I have no
idea how to do a --loadlast option since the data directory path is
initialized in the GUI code and references WX code.

You may then do slic3r.pl --load ~/.Slic3r/last_config.ini
whatever.stl to slice with your last configuration.

More of a suggestion than a final patch.