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/xsp/Flow.xsp

@@ -46,18 +46,6 @@ _new_from_width(CLASS, role, width, nozzle_diameter, height, bridge_flow_ratio)
     OUTPUT:
         RETVAL
 
-Flow*
-_new_from_spacing(CLASS, spacing, nozzle_diameter, height, bridge)
-    char*           CLASS;
-    float           spacing;
-    float           nozzle_diameter;
-    float           height;
-    bool            bridge;
-    CODE:
-        RETVAL = new Flow(Flow::new_from_spacing(spacing, nozzle_diameter, height, bridge));
-    OUTPUT:
-        RETVAL
-
 %}
 };