Add Units schemaTranslate for Unit::Velocity.

Independent of scale, use only mm/min or in/min because it is compatible
with GCODE feed rate units.
(My use case is definition os feed-rates in PathLoadTool.py).

src/Base/UnitsSchemaInternal.cpp

@@ -163,6 +163,10 @@ QString UnitsSchemaInternal::schemaTranslate(const Quantity &quant, double &fact
         unitString = QString::fromLatin1("W/m^2");
         factor = 1.0;
     }
+    else if (unit == Unit::Velocity) {
+        unitString = QString::fromLatin1("mm/min");
+        factor = 1.0/60;
+    }
     else {
         // default action for all cases without special treatment:
         unitString = quant.getUnit().getString();