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/src/slic3r/GUI/PresetHints.hpp

@@ -0,0 +1,25 @@
+#ifndef slic3r_PresetHints_hpp_
+#define slic3r_PresetHints_hpp_
+
+#include <string>
+
+#include "PresetBundle.hpp"
+
+namespace Slic3r {
+
+// GUI utility functions to produce hint messages from the current profile.
+class PresetHints
+{
+public:
+    // Produce a textual description of the cooling logic of a currently active filament.
+    static std::string cooling_description(const Preset &preset);
+    // Produce a textual description of the maximum flow achived for the current configuration
+    // (the current printer, filament and print settigns).
+    // This description will be useful for getting a gut feeling for the maximum volumetric
+    // print speed achievable with the extruder.
+    static std::string maximum_volumetric_flow_description(const PresetBundle &preset_bundle);
+};
+
+} // namespace Slic3r
+
+#endif /* slic3r_PresetHints_hpp_ */