WIP: Nullable configuration value concept, implemented for

ConfigOptionFloatsNullable, ConfigOptionIntsNullable,
ConfigOptionPercentsNullable, ConfigOptionBoolsNullable.

retract override values were added to the Filament profile:
vector of floats: "retract_length", "retract_lift", "retract_lift_above",
	"retract_lift_below", "retract_speed", "deretract_speed",
	"retract_restart_extra", "retract_before_travel",
vector of bools: "retract_layer_change", "wipe"
vector of percents: "retract_before_wipe"

src/boost/nowide/utf8_codecvt.hpp

@@ -102,7 +102,7 @@ protected:
         #ifndef BOOST_NOWIDE_DO_LENGTH_MBSTATE_CONST
         return from - save_from;
         #else
-        return save_max - max;
+        return int(save_max - max);
         #endif
     }