[PD] allow negative helix growth

Helices that become smaller with every turn are geometrically perfectly valid. Therefore we cannot forbid this.

(For example when creating a helix from a face you often cannot move it so that you can apply a positive growth.)

src/Mod/PartDesign/Gui/TaskHelixParameters.h

@@ -95,7 +95,7 @@ protected:
     App::PropertyBool*        propReversed;
     App::PropertyLinkSub*     propReferenceAxis;
     App::PropertyAngle*       propAngle;
-    App::PropertyLength*      propGrowth;
+    App::PropertyDistance*    propGrowth;
     App::PropertyEnumeration* propMode;
     App::PropertyBool*        propOutside;