Reworked the Perl unit / integration tests to use the same Print

interface that the application is using. Old interface used just
for the integration tests was removed.

lib/Slic3r.pm

@@ -50,7 +50,6 @@ use Slic3r::Point;
 use Slic3r::Polygon;
 use Slic3r::Polyline;
 use Slic3r::Print::Object;
-use Slic3r::Print::Simple;
 use Slic3r::Surface;
 our $build = eval "use Slic3r::Build; 1";