Initial definition of PresetHints C++ class and Perl binding, ported the cooling logic hints to C++.

Removed Perl Flow::new_from_spacing bindings.
Some Fill C++11 beautification.
Fix of a support_material_1st_layer_flow, brim_flow and skirt_flow logic to use the extrusion_width if both first_layer_extrusion_width and support_material_extrusion_width are undefined.
Documented the extrusion width logic in the config tooltips, including the default values.

xs/lib/Slic3r/XS.pm

@@ -179,14 +179,6 @@ sub new_from_width {
     );
 }
 
-sub new_from_spacing {
-    my ($class, %args) = @_;
-    
-    return $class->_new_from_spacing(
-        @args{qw(spacing nozzle_diameter layer_height bridge)},
-    );
-}
-
 package Slic3r::Surface;
 
 sub new {