Base: Quantity: use isDimensionless whenever feasible

Quantity is often queried for Unit just to see if it has a dimension.
Ask Quantity directly using isDimensionless() method and modify that
method not to care about Quantity value validity; no user was ever
asking for value validity.

src/Gui/QuantitySpinBox.cpp

@@ -224,7 +224,7 @@ public:
         ok = parseString(copy, res, value, path);
 
         // If result does not have unit: add default unit
-        if (res.getUnit().isEmpty()){
+        if (res.isDimensionless()) {
             res.setUnit(unit);
         }