[Draft] Fix to mirror tool

the normal is not computed with the view, but with the working plane if active.
ref. https://forum.freecadweb.org/viewtopic.php?f=23&t=44301&p=380632#p380370

src/Mod/Draft/Draft.py

@@ -2222,38 +2222,47 @@ def getCloneBase(obj,strict=False):
     return obj
 
 
-def mirror(objlist,p1,p2):
-    """mirror(objlist,p1,p2,[clone]): creates a mirrored version of the given object(s)
-    along an axis that passes through the two vectors p1 and p2."""
+def mirror(objlist, p1, p2):
+    """mirror(objlist, p1, p2)
+    creates a Part::Mirror of the given object(s), along a plane defined
+    by the 2 given points and the draft working plane normal.
+    """
 
     if not objlist:
-        FreeCAD.Console.PrintError(translate("draft","No object given")+"\n")
+        _err = "No object given"
+        FreeCAD.Console.PrintError(translate("draft", _err) + "\n")
         return
     if p1 == p2:
-        FreeCAD.Console.PrintError(translate("draft","The two points are coincident")+"\n")
+        _err = "The two points are coincident"
+        FreeCAD.Console.PrintError(translate("draft", _err) + "\n")
         return
     if not isinstance(objlist,list):
         objlist = [objlist]
 
+    if hasattr(FreeCAD, "DraftWorkingPlane"):
+        norm = FreeCAD.DraftWorkingPlane.getNormal()
+    elif gui:
+        norm = FreeCADGui.ActiveDocument.ActiveView.getViewDirection().negative()
+    else:
+        norm = FreeCAD.Vector(0,0,1)
+    
+    pnorm = p2.sub(p1).cross(norm).normalize()
+
     result = []
 
     for obj in objlist:
         mir = FreeCAD.ActiveDocument.addObject("Part::Mirroring","mirror")
-        mir.Label = "Mirror of "+obj.Label
+        mir.Label = "Mirror of " + obj.Label
         mir.Source = obj
-        if gui:
-            norm = FreeCADGui.ActiveDocument.ActiveView.getViewDirection().negative()
-        else:
-            norm = FreeCAD.Vector(0,0,1)
-        pnorm = p2.sub(p1).cross(norm).normalize()
         mir.Base = p1
         mir.Normal = pnorm
-        formatObject(mir,obj)
+        formatObject(mir, obj)
         result.append(mir)
 
     if len(result) == 1:
         result = result[0]
         select(result)
+
     return result